173 , No. 12 Cloning and Characterization of the Salmonella typhimurium ada Gene , Which Encodes 06-Methylguanine-DNA Methyltransferase ATSUSHI HAKURA ,1 KAZUSHIGE MORIMOTO ,2 TOSHIO SOFUNI ,1 AND TAKEHIKO NOHMII * Division of Genetics and Mutagenesis , Biological Safety Research Center , ' and Division of Chem-Bio Infjrmatic.s ,2 National Institute of Hygienic Sciences , 1-18-1 , Kamiyoga , Setagaya-ku , Tokyo 158 , Japan Received 8 February 1991/Accepted 5 April 1991 The ada gene of Escherichia coli encodes 06-methylguanine-DNA methyltransferase , which serves as a positive regulator of the adaptive response to alkylating agents and as a DNA repair enzyme .
The gene which can make an ada-deficient strain of E. coli resistant to the cell-killing and mutagenic effects of N-methyl-N ' - nitro-N-nitrosoguanidine ( MNNG ) has been cloned from Salmonella typhimurium TA1538 .
DNA sequence analysis indicated that the gene potentially encoded a protein with a calculated molecular weight of 39,217 .
Since the nucleotide sequence of the cloned gene shows-70 % similarity to the ada gene of E. coli and there is an ada box-like sequence ( 5 ' - GAATTAAAACGCA-3 ' ) in the promoter region , we tentatively refer to this cloned DNA as the adaST gene .
The gene encodes Cys-68 and Cys-320 , which are potential acceptor sites for the methyl group from the damaged DNA .
The multicopy plasmid carrying the adaST gene significantly reduced the frequency of mutation induced by MNNG both in E. coli and in S. typhimurium .
The AdaST protein encoded by the plasmid increased expression of the ada ' - lacZ chromosome fusion about 5-fold when an E. coli strain carrying both the fusion operon and the plasmid was exposed to a low concentration of MNNG , whereas the E. coli Ada protein encoded by a low-copy-number plasmid increased it about 40-fold under the same conditions .
The low ability of AdaST to function as a positive regulator could account for the apparent lack of an adaptive response to alkylation damage in S. typhimurium .
Methylating agents produce various methylated purine and pyrimidine adducts as well as forming methylphosphot-riesters ( 7 ) .
Among these adducts , 06-methylguanine ( O6 MeG ) is thought to be the most relevant adduct in terms of induction of mutations because this altered base directs the incorporation of either thymine or cytosine without blocking DNA replication , resulting in GC-to-AT transition mutation ( 3 , 18 , 54 ) .
This DNA adduct is mainly repaired by the 06-MeG-DNA methyltransferase ( MTase ) in many species , which directly transfers the methyl group from 06-MeG to one of its cysteine residues in an autoinactivating stoichiometric fashion ( 16 , 17 , 35 ) .
Escherichia coli cells have the ability to acquire increased resistance to the cell-killing and mutagenic effects of alkylating agents during exposure of cells to sublethal concentrations of the methylating and ethylating agents ( 11 , 44 ) .
This phenomenon has been termed the adaptive response to alkylating and requires functional ada which agents a gene , encodes a 39-kDa MTase ( 12 , 17 , 48 , 59 ) .
The Ada protein transfers the methyl group from 06-MeG and 04-methylth-ymine ( 04-MeT ) to its Cys-321 residue and also transfers the methyl group from one of the two stereoisomers of meth-ylphosphotriester ( the S configuration ) to its Cys-69 residue ( 4 , 21 , 22 , 23 , 35 , 55 , 63 ) .
The methylation at the Cys-69 residue converts the Ada protein into an efficient transcriptional activator of its own gene and other ( alkB , alkA , genes and aidB ) ( 15 , 30 , 32 , 52 , 55 , 60 , 61 ) .
The alkA gene product , 3-methyladenine DNA glycosylase II , efficiently repairs the potentially lethal lesions of N3-methylpurine and 02-meth-ylpyrimidine in DNA 31 ) .
Thus , the Ada protein plays ( 29 , an important role both as a repair enzyme and as a positive regulator in the adaptive The Ada protein is response .
composed of functional domains , each possessing two Cys-69 or Cys-321 , and is cleaved into the two domains by endogenous protease in E. coli ( 49 , 58 ) .
Interestingly , E. coli has a second MTase , which is a predominant MTase in unadapted E. coli ( 19 , 39 , 41 , 42 , 51 ) .
The second MTase , the Ogt protein , repairs the 06-MeG and 04-MeT but not the methylphosphotriesters and repairs the 06-ethylguanine in DNA at a higher rate than does the Ada protein ( 64 ) .
The amino-acid sequence of this 19-kDa protein shares homology with the C-terminal domain of the Ada protein ( 19 , 39 ) .
The expression of the ogt gene is constitutive ( 19 ) .
The ogt gene is located at 29 min on the E. coli chromosome map , whereas the ada gene is located at 47 min ( 38 , 47 ) .
In contrast to E. coli , the adaptive response to alkylating agents does not seem to occur in Salmonella typhimurium ( 8 , 9 , 17 ) .
Guttenplan and Milstein reported that levels of 06-MeG as well as mutagenesis induced by N-methyl-N ' - nitro-N-nitrosoguanidine ( MNNG ) are biphasic in S. typhihas a murium TA1535 and suggested that S. typhimurium saturable constitutive MTase ( 9 ) .
Although many derivatives of S. typhimurium have widely been used in the Ames test for the detection of environmental mutagens and carcinogens , including alkylating agents ( 20 ) , the molecular nature of its MTases has not been characterized .
To clarify the molecular basis of the apparent lack of the adaptive response in S. typhimurium , we have searched for the ada gene of S. typhimurium by screening for the gene which makes a Aada-25 strain of E. coli resistant to the cell-killing and mutagenic effects of MNNG .
Consequently , we have cloned the gene encoding the 39-kDa MTase , which shows 75 % similarity to the Ada protein of E. coli at the referred to the amino-acid level .
We have cloned tentatively gene as the adaST gene .
The gene product , AdaST , was found to be a weak activator of expression from the ada promoter of E. coli .
The accompanying report ( 60a ) provides biochemical evidence that the adasT gene in S. typhimurium is weakly induced by MNNG .
The low ability of AdaST to function as a positive regulator could account for the apparent lack of the adaptive response in S. typhimurium .
Possible factors which reduce the inducibility of the adaST gene in S. typhimurium are discussed .
366 MATERIALS AND METHODS Bacterial strains and plasmids .
All bacterial strains and plasmids used are described in Table 1 .
LB broth and agar were used for routine bacterial culture and supplemented , when necessary , with 50 pug of ampicillin per ml for plasmid selection and maintenance .
MNNG-containing agar plates contained the following : M9 salts ( 24 ) adjusted to pH 5.6 ; 0.2 % glucose ; 2 mM MgCl2 ; 0.1 mM CaCl2 ; 40 p , g each of threonine , leucine , arginine per ml ; 5 isoleucine , proline , valine , histidine , and of thiamine ml ; 1.5 , ug of MNNG per ml ; 50 , ug of , ug per ampicillin per ml ; and 1.5 % Bacto-Agar ( Difco ) .
Semien-riched-medium agar plates used for the reversion assay of argE3 to Arg + of E. coli were prepared as described previously ( 34 ) .
Vogel-Bonner minimal agar plates and top agar used for the reversion assay of hisG46 to His ' of S. typhimurium TA1535 were prepared as described previously ( 20 ) .
Supplemented M9-glucose medium for induction of the adaptive response of E. coli was prepared as described previously ( 34 ) except that it contained an increased amount of arginine ( 40 , ug/ml ) .
Histidine-fortified Vogel-Bonner me-dium for induction of the adaptive response of S. typhimu-rium was prepared as described previously ( 20 ) except that it contained histidine ( 50 , ug/ml ) and biotin ( 3 nmol/ml ) .
Cloning of the adaST gene encoding the MTase of S. typhimurium .
A gene library of S. typhimurium TA1538 was constructed by ligating partially Sau3AI-digested genomic DNA , whose size is about 10 kb , with BamHI-digested pBR322 ( 62 ) .
The library DNA was modified by introducing it into an XL1-blue strain ( rK MK + ) of E. coli .
More than 30,000 transformants of XL1-blue were collected , and plasmid DNA was extracted .
A Aada-25 strain of E. coli ( GW7101 ) was transformed with the modified library DNA , and the transformants were directly spread on MNNG-containing agar plates .
After incubation for 3 days at 37 °C , colonies that survived on the plates were selected .
Plasmid DNA was reisolated from such colonies and introduced into a fresh Aada-25 background .
The plasmid which makes a Aada-25 strain resistant to the mutagenic effect of MNNG was selected by checking the second transformants for their mutability to MNNG by a quantitative MNNG mutagenesis assay .
The selected plasmid ( pYG7001 ) was introduced into S. typhimurium TA1535 .
A Aada-25 strain containing pYG7001 was subjected to an assay for MTase enzyme activity .
Recloning of the adaST gene .
The genomic DNA of S. typhimurium TA1538 was with EcoRV and partially digested then subjected to 0.8 % agarose gel electrophoresis .
A band with an of about 2 kb was excised from the gel and ligated strain EcoRV-digested pBluescript KS + vector .
A Aada-25 of E. coli was transformed with the DNA , and ampicillin-resistant colonies were selected .
Each transformant was tested for resistance to the cell-killing effect of MNNG by streaking the cells on MNNG-containing agar plates .
Plasmid DNA was isolated from the master colonies of apparent MNNG-resistant transformants and subjected to restriction enzyme analysis with EcoRV , ClaI , NruI , and DraI .
The plasmid DNA ( pYG7034 ' ) which showed the same pattern of digestion as that of pYG7034 was selected and used for confirmation of the DNA sequence of the adaST gene .
Quantitative MNNG mutagenesis assay .
An aliquot ( 1 ml ) of the culture of log-phase cells was mixed with MNNG dissolved in dimethyl sulfoxide ( 10 , ul ) and incubated for 5 min at 37 °C with shaking .
The treated cells were washed twice with cold saline and resuspended .
A portion ( 0.1 ml ) of the cells was spread on semienriched-medium agar plates with a sterile glass rod ( E. coli ) or spread on Vogel-Bonner agar plates with 2 ml of molten top agar ( S. typhimurium ) .
For counting the surviving cells , the suspension was dilute J. BACTERIOL .
pSE101 is a derivative of pSC101 with the kanamycin resistance gene ( 6 ) .
pHSG576 is a derivative of pSC101 with the chloramphenicol resistance gene ( 56 ) .
b TA strains have the genotype A ( gal bio chl uvrB ) rfa .
c Detailed information of pYN3059 is in reference 28 .
pYG7034 ' pYG7070 pYN3059c pYG7050 pYG7090 105-fold with cold saline and 0.1 ml of the diluted suspension was spread on semienriched-medium agar plates or on Vogel-Bonner agar plates with top agar .
The induced-muta-tion frequency was calculated as described previously ( 34 ) .
The Ames test using S. typhimurium TA1535 and its deriv-atives was carried out as described previously ( 20 ) except that the cells treated with MNNG for 5 min at 37 °C were washed twice with cold saline before addition of the top agar .
All plates were incubated at 37 °C for 2 days .
The adaptive response was induced by exposing exponentialphase cultures ( A6 .
' of 0.3 to 0.4 ) to MNNG ( 1.5 , ug/ml ) for 1.5 h unless otherwise indicated .
Crude extracts were prepared by sonicating a culture as previously described ( 28 ) .
3H-labeled methyl-ated DNA was prepared by the reaction of calf thymus DNA ( Sigma Chemical Co. , St. Louis , Mo. ) with N - [ 3H ] methyl-N-nitrosourea ( 1.0 Ci/mmol ; Amersham , Buckinghamshire , United Kingdom ) ( 37 ) .
The extracts were assayed by incubation at 37 °C for 15 min of a reaction mixture containing 35 mM N-hydroxyethylpiperazine-N ' -2-ethanesulfonic-acid ( HEPES ) - KOH buffer ( pH 7.8 ) , 10 mM dithiothreitol , 500 , uM spermidine hydrochloride , 20 , ug of bovine serum albumin , 50 , ug of 3H-labeled methylated DNA containing 2.27 pmol of 06-MeG adduct , and the crude extracts in a total volume of 0.4 ml ( 25 ) .
After acid hydrolysis of a pelleted DNA , the hydrolysate was separated by high-performance liquid chromatography on a column of Whatman Partisil 1OSCX ( 4.6 by 250 mm ) .
The sample was eluted with 75 mM ammonium formate ( pH 4.0 ) at room temperature at a flow rate of 2 ml/min .
Each fraction was collected , and the radioactivity was determined in a liquid scintillation counter .
The retention times of 7-methylguanine , 06-MeG , and 3-methyladenine were 3.8 , 6.5 , and 13.2 min , respectively .
Labeling of plasmid-coded proteins in maxicells .
The maxicell method of Sancar et al. ( 45 ) was used to label the proteins encoded by plasmids pYG7034 and pYN3059 in strain CSR603 with [ 35S ] methionine .
Samples were run on a 12 % sodium dodecyl sulfate ( SDS ) - polyacrylamide gel and visualized by fluorography .
A set of deletion derivatives of pYG7034 was constructed by using the exonuclease III-mung bean nuclease digestion protocol from Stratagene .
Both strands of the 1.3-kb region containing the adasT gene were sequenced by the dideoxy-chain termination method ( 46 ) , using the Sequenase sequencing kit , version II ( U.S. Biochemical Corp. , Cleveland , Ohio ) .
DNA sequencing of pYG7034 ' was carried out as described above except that a set of synthetic oligonucleotide primers was used instead of creating the deletion derivatives .
Sequencing data were analyzed by using SDC-Genetyx software ( SDC Software Development Co. , Tokyo , Japan ) .
The 3-galactosidase assay was performed as described by Miller ( 24 ) .
MNNG ( 1 , ug/ml ) was added to the cells ( A6 .
of 0.2 ) of derivatives of GW5354 grown in supplemented M9-glucose medium and remained in the samples throughout the experiments ( 53 ) .
P-Galactosi-dase activity was assayed 2 h after the addition of MNNG .
Nucleotide sequence accession number .
The nucleotide sequence data reported here will appear in the DDBJ , EMBL , and GenBank nucleotide sequence data bases under accession number D90221 .
RESULTS Apparent lack of the adaptive response to MNNG in S. typhimurium TA1535 .
To validate the notion that S. typhimu-rium lacks the adaptive response to alkylating agents , we compared the response of S. typhimurium TA1535 to suble-thal MNNG treatment with those of an E. coli AB11S7 Aada-25 strain ( GW7101 ) and of E. coli AB1157 .
The MNNG-induced mutation frequencies of S. typhimu-rium TA1535 as well as of a Aada-25 strain were not decreased but rather increased by pretreatment of the cells with 0.3 , ug of MNNG per ml for 1.5 h ( Fig. 1A and B ) .
Pretreatment with lower ( 0.0375 , 0.075 , and 0.15 p.g / ml ) or higher ( 0.5 and 0.7 , ug/ml ) concentrations of MNNG did not induce the adaptive response in S. typhimurium TA1535 ( data not shown ) .
In contrast , the MNNG-induced mutation frequencies of strain AB1157 were significantly reduced by pretreatment of the cells with MNNG , as expected ( Fig. 1C ) .
The induced-mutation frequency was lowered about 50-fold by the pretreatment when the frequencies were compared at a challenging dose of 8 , ug of MNNG per ml .
From these results , we concluded that S. typhimurium TA1535 actually lacks the adaptive response to MNNG , and we investigated the molecular basis of this lack by cloning the ada gene of S. typhimurium .
Cloning of the gene encoding the MTase of S. typhimurium A10 1000 r B C 100 r 0 .
c 0 ) a0o ) C ) 100 F U ) 0 1 CD Q 0 0 ~ -1 ( n U ) CU c a ) a1 ) 0.1 k cc a : 00 ) ) fU .
0.1 0.1 1 10 0.1 1 10 * 0 0.1 1 10-MNNG ( upg/ml ) MNNG ( , ug/ml ) MNNG ( , ug/ml ) FIG. 1 .
Induced-mutation frequencies by MNNG in S. typhimurium TA1535 ( A ) , E. coli AB1157/ada -25 ( B ) , and E. coli AB1157 ( C ) .
Cells were adapted with 0.3 , ug of MNNG per ml for 1.5 h and then challenged with different concentrations of MNNG .
The reversion frequencies of hisG46 to His ' ( S. typhimurium ) or of argE3 to Arg + ( E. coli ) were determined .
Closed and open symbols represent the frequencies of the adapted and unadapted cells , respectively .
aoC ) 10 a ) 1 c To clone the adaST gene , we have screened for the gene which makes a Aada-25 strain resistant to the cell-killing and mutagenic effects of MNNG .
We obtained 10 candidates which apparently showed resistance to the cell-killing effect of MNNG by selecting the transformants of a Aada-25 strain with the library DNA of S. typhimurium TA1538 on MNNG-containing agar plates .
About 10,000 ampicillin-resistant colonies appeared when the transformants of a Aada-25 strain with the same amount of the library DNA were incubated on the same agar plates without MNNG .
To confirm that this resistance is due to the presence of the plasmids carrying a part of the chromosome DNA of TA1538 , we isolated the plasmids from the resistant colo-nies , introduced them into a fresh Aada-25 background , and then performed a quantitative MNNG mutagenesis assay .
One plasmid , which we designated pYG7001 , made the host strain resistant to the mutagenic and cell-killing effects of MNNG .
A Aada-25 strain containing pYG7001 showed about 10 times lower mutability to MNNG than did a Aada-25 strain containing pBR322 ( Fig. 2A ) .
Since other plasmids did not make a Aada-25 strain resistant to the mutagenic effect of MNNG , we did not study them further .
A Aada-25 strain containing pYG7001 showed higher resistance to the cell-killing effect of MNNG than did the control strain ( Fig. 2B ) .
Plasmid pYG7001 was transferred to S. typhimurium TA1535 .
The resulting transformant , S. typhi-murium TA1535 containing pYG7001 , also showed about 10 times lower mutability toward MNNG than did TA1535 containing pBR322 ( Fig. 2C ) .
To confirm that the cloned plasmid pYG7001 encodes the MTase , we assayed the MTase activity of the crude extract of a Aada-25 strain containing pYG7001 , using high-performance liquid chroma-tography .
The extract showed activity of more than 1.1 pmol of 06-MeG removed per mg of protein .
From these results , we have concluded that plasmid pYG7001 encodes the MTase of S. typhimurium .
Determination of the minimum region of cloned DNA essential for MTase activity .
A restriction map of pYG7001 was constructed by digesting the plasmid with several restriction enzymes ( Fig. 3 ) .
The plasmid ( 9.9 kb ) was composed of pBR322 ( 4.4 kb ) and genomic DNA of TA1538 ( 5.5 kb ) .
To determine the region necessary for encoding the MTase , we partially digested pYG7001 with EcoRV and constructed a set of deletion derivatives .
Upon checking the sensitivities of Aada-25 strains containing such deletion plasmids on MNNG-containing agar plates , we concluded that the MTase is encoded in the 2.0-kb region spanning from map position 2.5 kb ( the second EcoRV site ) to 4.5 kb ( the fifth EcoRV site ) of pYG7001 .
The restriction map within the 2.0-kb region was different from those of the ada and ogt genes of E. coli .
To further define the minimum DNA region necessary for encoding the MTase , we subcloned the 2.0-kb DNA fragment into an EcoRV site of the pBluescript KS + vector .
The resulting plasmid , pYG7034 , efficiently made a Aada-25 strain resistant to the killing effect of MNNG ( data not shown ) .
More than 50 deletion derivatives were prepared from both sides of the insert DNA of pYG7034 according to the exonuclease III-mung bean nuclease digestion protocol .
Among these deletion plasmids , 24 subcloned plasmids were introduced into a Aada-25 strain and checked for the ability to make the host strain resistant to the cell-killing effect of MNNG , using MNNG-containing agar plates .
The four longest subclones whose insert DNA was deleted from the EcoRV site on the right in Fig. 4 could complement a Aada-25 strain , while the plasmids which had about a 0.7-kb deletion from the same site no longer complemented the ada mutation .
In the same way , the four longest subclones whose insert DNA was deleted from the EcoRV site on the left in Fig. 4 could complement the ada mutation , whereas the plasmids whose insert DNA was deleted more than about 0.6 kb from the same site no longer complemented the mutation .
Thus , we concluded that the 1.3-kb region shown as an open box in Fig. 4 is the minimum region essential for encoding the MTase of S. typhimurium .
Identification of the MTase of S. typhimurium .
To identify the MTase of S. typhimurium , proteins synthesized in maxicells containing pYG7034 and pYN3059 were labeled with [ 35S ] methionine and were subjected to SDS-polyacrylamide gel electrophoresis and fluorography .
Plasmid pYG7034 produced a protein with an approximate molecular mass of 38 kDa ( Fig. 5 ) .
The minor band of a protein with an approximate molecular mass of 15 kDa was also observed .
Plasmid pYN3059 carrying the E. coli ada gene produced a protein with a molecular mass of 39 kDa , as expected .
No bands around the molecular mass of 19 kDa corresponding to that of the Ogt protein of E. coli were observed .
Thus , we suggest that the MTase encoded by the cloned DNA is analogous to the E. coli Ada protein but not to the Ogt protein and tentatively refer to the cloned gene as adaST .
DNA sequence of the adaST gene .
We have determined the nucleotide sequences of both strands of the 1.3-kb region which carries the adasT gene ( Fig. 6 ) .
Since the first cloned plasmid , pYG7001 , was selected on MNNG-containing agar plates , it was possible that mutations could arise in the cloned gene .
To avoid such a complication , we recloned the DNA which was not exposed to MNNG and confirmed that the 1.3-kb region shown in Fig. 6 has no mutational changes induced by MNNG .
The nucleotide sequence of the 1.3-kb region contains an open reading frame ( ORF ) of 1,056 bp and part of the other ORF spanning from nucleotides 1192 to to the 5 ' of the alkB gene which is region 1266 , homologous of E. coli ( 14 ) .
We did not determine the nucleotide sequence of the rest of the 3 ' region of the latter ORF .
The former ORF potentially encoded a protein with a calculated molecular weight of 39,217 , and the nucleotide sequence showed 70 % similarity to the ada gene of E. coli .
The similarity at the amino-acid level between the two gene products was 75 % .
Thus , we suggest that the former ORF is the coding region of the adaST gene and the latter is that of the alkB-like gene of S. typhimurium .
The termination codon of adaST and the initiation codon of the alkB-like gene were separated by 2 bp , whereas the ada and alkB genes of E. coli overlap by 1 bp ( 14 ) .
Upstream of the adaST gene , there was a potential ada box sequence ( 5 ' - GAATTAAAACGCA-3 ' ) , which differs from the E. coli ada box ( 5 ' - AAANNAAAGCGCA ) by 2 bp ( Fig. 7 ) .
Possible -35 , -10 , and Shine-Dalgarno regions were also observed .
The distance between the -10 region and the Shine-Dalgarno region of the adaST gene was 10 bp longer than that of the E. coli ada gene .
The amino-acid sequence homology between AdaST and E. coli Ada was not restricted to a specific region but spanned all of the coding region ( Fig. 8 ) .
E. coli and S. typhimurium are about 80 % homologous in DNA sequence ( 40 ) .
The N-terminal halves of the AdaST and the E. coli Ada proteins shared homology with the AdaA protein of Bacillus subtilis ( 27 ) , whereas the C-terminal halves were homologous to the AdaB and Dat proteins of B. subtilis ( 13 , 26 , 27 ) , the Ogt protein of E. coli ( 39 ) , and the human MTase ( MGMT ) ( 10 , 57 ) .
The adasT gene encodes Cys-68 and Cys-320 , which are highly conserved in other MTases and are possible acceptor sites for the methyl groups from the damaged DNA .
The gene also encodes the sequence of Lys-178 and Glu-179 , which corre sponds to a main cleavage site by an endogenous protease in E. coli .
Low ability of AdaST to act as a transcriptional activator .
To characterize the ability of AdaST to function as a transcriptional activator , we introduced plasmid pYG7001 carrying the adaST gene into a strain of E. coli containing a chromosomal ada ' - lacZ fusion integrated at the ada locus ( GW5354 ) ( 53 ) .
In this strain , the ada is joined to the lacZ gene gene so that levels of expression of the readily be detergene can mined simply by assaying,-galactosidase activity .
As shown in Table 2 , ada transcription was induced about fivefold by MNNG if the cells harbored pYG7001 carrying the adaST gene .
In contrast , ada transcription was induced more than 40-fold by MNNG if the cells had pYG7050 , a low-copy-number plasmid carrying the E. coli ada gene .
To more accurately compare the effects of the AdaST and E. coli Ada proteins on ada transcription , we subcloned the adaST gene and the E. coli ada gene onto the same low-copy-pBR322 107 101 ( 0.9 ) pHSG576 84 73 ( 0.9 ) pYG7001 adaST pBR322 187 855 ( 4.6 ) pYG7050 ada pSE101 72 3,069 ( 42.6 ) pYG7070 adaST pHSG576 111 1,413 ( 12.7 ) pYG7090 ada pHSG576 93 3,609 ( 38.8 ) a Detailed descriptions of pYG7001 , pYG7070 , pYG7090 , and pYG7050 are given in Table 1 .
b pBR322 is a high-copy-number plasmid .
Both pHSG576 and pSE101 are low-copy-number plasmids .
c Expressed as units per optical density at 600 nm ( 24 ) .
Numbers in parentheses represent relative values when the activities of uninduced cells are assigned a value of 1.0 .
P-Galactosidase assays were performed on strains containing a chromosome ada ' - lacZ fusion ( GW5354 ) and a plasmid carrying the adasT or the E. coli ada gene ( 53 ) .
Induced cultures were assayed 2 h after the addition of 1 , ug of MNNG per ml .
Uninduced cultures were assayed after 2 h of growth without MNNG number plasmid , pHSG576 .
The resulting plasmids , pYG7070 and pYG7090 , were introduced into strain GW5354 , and P-galactosidase activities were determined .
As shown in Table 2 , ada transcription was induced about 13-fold by MNNG if the cells had pYG7070 carrying the adaST gene , while ada transcription was induced about 40-fold by MNNG if the cells had pYG7090 carrying the E. coli ada gene .
From these results , we suggest that AdaST can activate ada transcription in E. coli to some extent but that its ability to function as a transcriptional activator is much lower than that of the E. coli Ada protein .
A B C 1000 r 0 10000 0 : 0 ) D ~ ~ ~ 100p 2 ` an ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ a. CaC ) m 1000 ( D - a 0 co a D 0 ) 100 c 101 ¬ 0 ) C ) - o 0.01 L 0.1 1 10 0 10 20 30 40 50 10 100 0 1 MNNG ( gg/ml ) MNNG ( gg/mI ) MNNG ( ugfml ) FIG. 2 .
Effects of pYG7001 on the mutagenesis and cell killing induced by MNNG in E. coli AB1157 Aada-25 ( A and B ) and on the mutagenesis induced by MNNG in S. typhimurium TA1535 ( C ) .
Symbols : 0 , E. coli AB1157 Aada-25 containing pYG7001 ; 0 , E. coli AB1157 Aada-25 containing pBR322 ; * , S. typhimurium TA1535 containing pYG7001 ; c , S. typhimurium TA1535 containing pBR322 .
resistance to MNNG + + + t > > > a a adasT resistance to MNNG Ori + Sal 1 ( 6 .
Partial restriction map of the 2.0-kb EcoRV-EcoRV region of pYG7034 and predicted location of the adasT gene .
Bars represent DNA of the 2.0-kb region remaining after exonuclease III-mung bean nuclease digestion .
+ and-indicate the ability and inability , respectively , of each subclone to make E. coli AB1157 Aada-25 resistant to the cell-killing effect of MNNG .
Resistance of each transformant was checked by using MNNG-containing agar plates FIG. 3 .
Restriction map of pYG7001 .
The EcoRI restriction site derived from pBR322 was assigned map position 0 kb of the 9.9-kb pYG7001 map .
A DNA fragment ( 5.5 kb ) derived from S. typhimu-rium TA1538 was inserted into the BamHI site of pBR322 .
adaST was located in a 2.0-kb EcoRV-EcoRV fragment ( E ) which was used for subcloning into pBluescript KS + .
Ori , DNA replication origin ; Ap , ampicillin resistance gene .
2 1 97.4 kD ¬ 66.2 kD ¬ 42.7 kD E. coli Ada AdaST 31 .
OkD ¬ A-lactamase 21.5 kD ¬ 14.4 kD ¬ FIG. 5 .
[ 35S ] methionine-labeled proteins synthesized in maxicells containing pYG7034 carrying the adaST gene ( lane 1 ) and pYN3059 carrying the E. coli ada gene ( lane 2 ) .
Nucleotide and predicted amino-acid sequences of the adaST gene .
An ORF corresponding to the adaST gene extends from nucleotides +131 to +1186 .
The putative ada box , ribosome-binding site ( Shine-Dalgarno [ S.D. ] sequence ) , and -35 and -10 regions are underlined .
Nucleotide and predicted amino-acid sequences of the 5 ' region of the alkB-like gene extending from nucleotide + 1192 are also presented ~ ~ GCA'ITAGr adas : ada box -35 -10 S.D. ada AGGAAAArAAcCAAGA ada box -35 JTACGGCAGCcrAGCrTarACAGGAGCGTI -10 S.D. alkA : -10 S.D. ada box -35 FIG. 7 .
Promoter region of the adasT , E. coli ada , and alkA genes .
The conserved ada box sequence , proposed -35 and -10 regions , and Shine-Dalgarno ( S.D. ) sequence are underlined .
The transcription initiation site is represented by a dot .
The initiation codon ( ATG ) is represented by capital letters .
Homology among the S. typhimurium ( AdasT ) , E. coli ( Ada and Ogt ) , B. subtilis ( AdaA , AdaB , and Dat ) , and human ( MGMT ) MTases .
Amino acid residues that are conserved between AdaST and at least one of other MTases are boxed .
* , possible methyl acceptor sites from alkylated-DNA ; , amino-acid sequences of the AdaA and Ada proteins that are similar to the sequence Glu-Ser-Val-Ala-Asp of the lambda CI repressor .
Effect of AdaST on ada transcription 1-Galactosidase activityc Uninduced Induced PlasmidaasmRelevant Parent gene vectorb DISCUSSION Although an adaptive response to alkylation damage has been observed in a number of microorganisms , several reports suggest that this response does not occur in S. typhimurium ( 8 , 9 , 17 ) .
Since chromosome DNA sequences that hybridize with the E. coli ada gene have been detected in S. typhimurium ( 50 ) , it was assumed that S. typhimurium had the ada-like gene but that its function might be much weaker than that of the E. coli ada gene ( 17 ) .
To clarify the molecular nature of the ada gene of S. typhimurium and its relation to the apparent lack of the adaptive response of this microorganism , we have searched for this gene by screening for the gene which makes a Aada-25 strain of E. coli resistant to the cell-killing and mutagenic effects of MNNG .
DNA sequence analysis indicated that the cloned gene encodes a protein with a molecular mass of 39 kDa , which is consistent with the value deduced from the [ 35S ] methionine-labeling experiments using the maxicell technique ( Fig. 5 ) .
The 15-kDa protein identified in maxicells containing pYG7034 might be produced by the alkB-like gene of S. typhimurium ( Fig. 6 ) .
Since the molecular weight of the E. coli AlkB protein is 23,900 ( 14 ) , we have apparently cloned only a fragment of the alkB-like gene of S. typhimurium .
The DNA sequence of the cloned gene encoding the 39-kDa protein shows-70 % similarity to the E. coli ada gene at the nucleo-tide level , and there was an ada box-like sequence upstream of the gene ( Fig. 6 to 8 ) .
Thus , we conclude that the gene we have cloned is an analog of the E. coli ada gene and tentatively refer to the cloned gene as adaST .
The multicopy plasmid carrying the adaST gene ( pYG 7001 ) significantly reduced the mutation frequency induced by MNNG in E. coli and S. typhimurium ( Fig. 2A and C ) .
These results suggest that the AdaST protein is functional at least as a repair enzyme in E. coli and in S. typhimurium .
In fact , the crude extract of a Aada-25 strain containing pYG7001 showed detectable repair activity for 06-MeG .
In accordance with these results , the predicted amino-acid sequence of AdaST contains the sequence Pro-Cys-His-Arg-Val around Cys-320 ( Fig. 6 ) .
This amino-acid sequence is highly conserved in all MTases whose genes have been cloned and sequenced ( Fig. 8 ) .
Cys-321 of the E. coli Ada protein , which corresponds to Cys-320 of the AdaST protein , is regarded as a methyl acceptor site from 06-MeG and 04-MeT in damaged DNA ( 17 , 52 , 55 ) .
In the accompanying report ( 60a ) , Vaughan and Sedgwick report that the AdaST protein repairs 06-MeG in DNA as efficiently as does the E. coli Ada protein .
In contrast to the DNA repair activity , the ability of the AdaST protein to function as a transcriptional activator was much lower than that of the E. coli Ada protein ( Table 2 ) .
The AdaST protein encoded by pYG7001 increased expression of the ada ' - lacZ chromosome fusion about 5-fold when an E. coli strain carrying both the fusion operon and the plasmid was exposed to a low concentration of MNNG , whereas the Ada protein of E. coli encoded by pYG7050 increased it about 40-fold under the same conditions .
Even with the same copy number , the AdaST protein encoded by pYG7070 showed a much smaller effect on ada transcription than did the E. coli Ada protein encoded by pYG7090 ( Table 2 ) .
Vaughan and Sedgwick ( 60a ) directly measured the extent of induction of the adaST gene in S. typhimurium by using the immunoblotting technique and reported that the 39-kDa protein was weakly induced by pretreating the S. typhimurium cells with MNNG .
The low ability of AdaST to function as a transcriptional activator could account at least in part for the apparent lack of the adaptive response to alkylating agents in S. typhimurium ( Fig. 1 ) .
In unadapted cells of E. coli , about 30 to 40 molecules of MTase are expressed per cell , and more than 95 % of them constitute the 19-kDa protein , probably the Ogt protein ( 19 , 42 ) .
On the other hand , Rebeck et al. identified an active 19-kDa MTase in S. typhimurium and estimated that about 30 molecules of this MTase are constitutively expressed per cell ( 42 ) .
Morohoshi and Munakata also identified a similarsize MTase in unadapted cells of S. typhimurium ( 27a ) .
These results raise the possibility that S. typhimurium has the Ogt-like protein and that this form of MTase is predominant in unadapted cells .
If so , the apparent lack of the adaptive response of S. typhimurium would be due to the low concentration of the AdaST protein in unadapted cells as well as to its low ability to function as a transcriptional activator .
Why is the extent of induction of the adaST gene in S. typhimurium much lower than that of induction of the ada gene in E. coli ?
The following are possible factors which reduce the inducibility of the adaST gene in S. typhimurium .
( i ) In the E. coli Ada protein , transfer of the methyl group from the methylphosphotriester to Cys-69 converts Ada from a weak to a strong DNA-binding protein and transcriptional activator ( 17 , 42 , 52 ) .
If AdaST had a poor repair activity to the methylphosphotriester , it would have a low ability to convert itself to a transcriptional activator .
This possibility , however , seems to be unlikely since adasT encodes the sequence Phe-Arg-Pro-Cys-Lys-Arg-Cys , which is conserved in the E. coli Ada and B. subtilis AdaA proteins ( Fig. 8 ) .
Cys-68 at the fourth position of the sequence corresponds to Cys-69 of the E. coli Ada protein .
In fact , Vaughan and Sedgwick ( 60a ) report that the AdaST protein shows considerable repair activity to the meth-ylphosphotriester .
( ii ) If the promoter sequence of the adaST gene diverged greatly from the corresponding sequences of the E. coli ada and alkA genes , it would have a low ability to interact with a transcriptional activator .
The nucleotide sequence of the promoter region of the adaST gene is , however , fairly similar to that of the region of the E. coli ada gene ( Fig. 7 ) .
The ada box-like sequence ( 5 ' -- QAATTAAAACGCA-3 ' ) of adaST is identical to the ada box sequence ( 5 ' - AAANNAAA-iCGC A-3 ' ) of the E. coli ada and alkA genes ( 52 ) except for two nucleotides , i.e. , the first G and the ninth A of the ada boxlike sequence .
However , Nakamura et al. reported that removal of the first five nucleotides of the ada box sequence or the change of G to A at the ninth position of the sequence had little effect on expression of the E. coli ada gene ( 33 ) .
If a similar regulation mechanism exists in S. typhimurium , changes of these nucleotides will not seriously affect the function of the ada box-like sequence of the adaST gene .
The only marked difference in the DNA sequence of the promoter region between the adaST and E. coli ada genes is tha the adaST has an extra 10-bp sequence at the 3 ' side of the -10 region ( Fig. 7 ) .
Since the size of 10 bp corresponds to a full turn of the DNA helix , it would be interesting to determine the actual initiation site of transcription of the adaST .
( iii ) Changes of amino-acids might directly affect the some ability of the AdaST protein to bind the specific DNA sequence or to activate the RNA polymerase .
Studies on the E. coli Ada protein suggest that the region essential for the specific DNA binding resides on the N-terminal half of the Ada protein ( 1 ) .
Recently , Morohoshi et al. reported that a portion of the AdaA protein of B. subtilis spanning from amino-acids 117 to 137 has the potential to form a helix-turn-helix motif ( 27 , 36 ) .
Dodd and Egan suggested that theE .
coli Ada protein also has a potential helix-turn-helix motif at the corresponding region ( 5 ) .
The AdaA protein shows similarity to the lambda CI repressor , which is a typical DNA-binding protein ( 27 , 36 ) .
The sequence Glu-Ser-Leu-Ala-Asp , which is present in the AdaA protein from amino-acids 119 to 123 , is almost identical to the corresponding sequence , Glu-Ser-Val-Ala-Ap , of the repressor .
Of the five amino-acid resi-dues of the sequences , two acidic residues , the first Glu and the fifth Asp , are regarded as crucial for the ability of the lambda CI activator repressor to function as a transcriptional ( 2 ) .
Interestingly , AdaST contains the sequence Ala-Phe-Leu-Ala-Gln from amino-acids 102 to 106 , whereas the E. coli Ada protein has the sequence Glu-Ala-Leu-Ala-Asp at the corresponding region ( Fig. 8 ) .
Thus , it might be interesting to determine the effect of changes of the first Ala and the fifth Gln residues of the AdaST sequence to acidic residues on the ability of the protein to function as a transcriptional activator .
We thank P. Vaughan , B. Sedgwick , F. Morohoshi , and N. Munakata for sharing observations prior to publication .
We are grateful to G. C. Walker and M. Sekiguchi for helpful advice and for providing bacterial strains and plasmids .
We also thank M. Yamada and C. R. Warner for critically reading the manuscript .
This work was partly supported by a grant-in-aid from the Japan Health Science Foundation .
A.H. was supported by a fellowship from the Japan Health Science Foundation .
Akimaru , H. , K. Sakumi , T. Yoshikai , M. Anai , and M. Sekigu-chi .
Positive and negative regulation of transcription by a cleavage product of Ada protein .
Bushman , F. D. , C. Shang , and M. Ptashne .
A single glutamic-acid residue plays a key role in the transcriptional activation function of lambda repressor .
Coulondre , C. , and J. H. Miller .
Genetic studies of the lac repressor .
Mutagenic specificity in the lacI gene of Esche-richia coli .
Demple , B. , B. Sedgwick , P. Robins , N. Totty , M. D. Waterfield , and T. Lindahl .
Active site and complete sequence of the suicidal methyltransferase that counters alkylation mutagenesis .
Dodd , I. B. , and J. B. Egan .
Improved detection of helix-turn-helix DNA-binding motifs in protein sequences .
Elledge , S. J. , and G. C. Walker .
Proteins required for ultraviolet light and chemical mutagenesis .
W.H. Freeman & Co. , New York .
Comutagenic effects exerted by N-ni-troso compounds .
Guttenplan , J. B. , and S. Milstein .
Resistance of Salmonella typhimurium TA1535 to 06-guanine methylation and mutagen ¬ esis induced by low doses of N-methyl-N ' - nitro-N-nitrosogua-nidine : an apparent constitutive repair activity .
Hayakawa , H. , G. Koike , and M. Sekiguchi .
Expression and cloning of complementary DNA for a human enzyme that repairs 06-methylguanine in DNA .
Jeggo , P. , M. Defais , L. Samson , and P. Schendel .
An adaptive response of E. coli to low levels of alkylating agent : comparison with previously characterized DNA repair pathways .
Karran , P. , T. Lindahl , and B. Griffin .
Adaptive response to alkylating agents involves alteration in situ of 06-methylgua-nine residues in DNA .
Kodama , K. , Y. Nakabeppu , and M. Sekiguchi .
Cloning and expression of the Bacillus subtilis methyltransferase gene in Escherichia coli ada-cells .
Kondo , H. , Y. Nakabeppu , H. Kataoka , S. Kuhara , S. Kawabata , and M. Sekiguchi .
Structure and expression of the alkB gene of Escherichia coli related to the repair of alkylated-DNA .
LeMotte , P. K. , and G. C. Walker .
Induction and auto-regulation of ada , a positively acting element regulating the response of Escherichia coli K-12 to methylating agents .
Lindahl , T. , B. Demple , and P. Robins .
Suicide inactivation of the E. coli 06-methylguanine-DNA methyltransferase .
Lindahl , T. , B. Sedgwick , M. Sekiguchi , and Y. Nakabeppu .
Regulation and expression of the adaptive response to alkylating agents .
Loechler , E. L. , C. L. Green , and J. M. Essigmann .
In vivo mutagenesis by 06-methylguanine built into a unique site in a viral genome .
Margison , G. P. , D. P. Cooper , and P. M. Potter .
The E. coli ogt gene .
Maron , D. M. , and B. N. Ames .
Revised methods for the Salmonella mutagenicity test .
McCarthy , J. G. , B. V. Edington , and P. F. Schendel .
Inducible repair of phosphotriesters in Escherichia coli .
McCarthy , T. V. , P. Karran , and T. Lindahl .
Inducible repair of O-alkylated-DNA pyrimidines in Escherichia coli .
McCarthy , T. V. , and T. Lindahl .
Methyl phosphotriesters in alkylated-DNA are repaired by the Ada regulatory protein of E. coli .
Experiments in molecular genetics .
Cold Spring Harbor Laboratory , Cold Spring Harbor , N.Y. 25 .
Morimoto , K. , M. E. Dolan , D. Scicchitano , and A. E. Pegg .
Repair of 06-propylguanine and 06-butylguanine in DNA by 06-alkylguanine-DNA-alkyltransferase from rat liver and E. coli .
Morohoshi , F. , K. Hayashi , and N. Munakata .
Bacillus subtilis gene coding for constitutive 06-methylguanine-DNA alkyltransferase .
Morohoshi , F. , K. Hayashi , and N. Munakata .
Bacillus subtilis ada operon encodes two DNA alkyltransferases .
Morohoshi , F. , and N. Munakata .
Nakabeppu , Y. , H. Kondo , S. Kawabata , S. Iwanaga , and M. Sekiguchi .
Purification and structure of the intact Ada regulatory protein of Escherichia coli K12 , 06-methylguanine-DNA methyltransferase .
Nakabeppu , Y. , H. Kondo , and M. Sekiguchi .
Cloning and characterization of the alkA gene of Escherichia coli that encodes 3-methyladenine DNA glycosylase II .
Nakabeppu , Y. , Y. Mine , and M. Sekiguchi .
Regulation of expression of the cloned ada gene in Escherichia coli .
Nakabeppu , Y. , T. Miyata , H. Kondo , S. Iwanaga , and M. Sekiguchi .
Structure and expression of the alkA gene of Escherichia coli involved in adaptive response to alkylatin 32 .
Nakabeppu , Y. , and M. Sekiguchi .
Regulatory mechanisms for induction of synthesis of repair enzymes in response to alkylating agents : Ada protein acts as a transcriptional regulator .
Nakamura , T. , Y. Tokumoto , K. Sakumi , G. Koike , Y. Naka-beppu , and M. Sekiguchi .
Expression of the ada gene of Escherichia coli in response-to alkylating agents : identification of transcriptional regulatory elements .
Nohmi , T. , A. Hakura , Y. Nakai , M. Watanabe , S. Y. Murayama , and T. Sofuni .
Salmonella typhimurium has two homologous but different umuDC operons : cloning of a new umuDC-like operon ( samAB ) present in a 60-megadalton cryptic plasmid of S. typhimurium .
Olsson , M. , and T. Lindahl .
Repair of alkylated-DNA in Escherichia coli : methyl group transfer from 06-methylguanine to a protein cysteine residue .
Pabo , C. O. , and R. T. Sauer .
Pegg , A. E. , W. Perry , and R. A. Bennett .
Effect of partial hepatectomy on removal of 06-methylguanine from alkylated-DNA by rat liver extracts .
Potter , P. M. , L. Harris , and G. P. Margison .
Mapping of Ogt in the E. coli chromosome .
Potter , P. M. , M. C. Wilkinson , J. Fitton , F. J. Carr , J. Brennand , D. P. Cooper , and G. P. Margison .
Characterization and nucleotide sequence of ogt , the 06-alkylguanine-DNA-alkyltransferase gene of E. coli .
Rayssiguier , C. , D. S. Thaler , and M. Radman .
The barrier to recombination between Escherichia coli and Salmonella typhimurium is disrupted in mismatch-repair mutants .
Rebeck , G. W. , S. Coons , P. Carroll , and L. Samson .
A second DNA methyltransferase repair enzyme in Escherichia coli .
Rebeck , G. W. , C. M. Smith , D. L. Goad , and L. Samson .
Characterization of the major DNA repair methyltransferase activity in unadapted Escherichia coli and identification of a similar activity in Salmonella typhimurium .
Sakumi , K. , and M. Sekiguchi .
Regulation of expression of the ada gene controlling the adaptive response : interactions with the ada promoter of the Ada protein and RNA polymerase .
Samson , L. , and J. Cairns .
A new pathway for DNA repair in Escherichia coli .
Sancar , A. , A. M. Hack , and W. D. Rupp .
Simple method for identification of plasmid-coded proteins .
Sanger , F. , S. Nicklen , and A. R. Coulson .
DNA sequencing with chain-terminating inhibitors .
Genetic mapping of ada and adc mutations affecting the adaptive response of Escherichia coli to alkylating agents .
Molecular cloning of a gene which regulates the adaptive response to alkylating agents in Escherichia coli .
Sedgwick , B. , P. Robins , N. Totty , and T. Lindahl .
Functional domains and methyl acceptor sites of the Esche-richia coli Ada protein .
Sedgwick , S. G. , and P. A. Goodwin .
Differences in mutagenic and recombinational DNA repair in enterobacteria .
Shevell , D. E. , A. M. Abou-Zamzam , B. Demple , and G. C. Walker .
Construction of an Escherichia coli K-12 ada deletion by gene replacement in a recD strain reveals a second methyltransferase that repairs alkylated-DNA .
Shevell , D. E. , B. M. Friedman , and G. C. Walker .
Resistance to alkylation damage in Escherichia coli : role of the Ada protein in induction of the adaptive response .
Shevell , D. E. , P. K. Lemotte , and G. C. Walker .
Altera-tion of the carboxy-terminal domain of Ada Protein influences its inducibility , specificity , and strength as a transcriptional activator .
Snow , E. T. , R. S. Foote , and S. Mitra .
Base-pairing properties of 06-methylguanine in template DNA during in-vitro DNA replication .
Takano , K. , Y. Nakabeppu , and M. Sekiguchi .
Functional sites of the Ada regulatory protein of Escherichia coli ; analysis by amino-acid substitutions .
Takeshita , S. , M. Sato , M. Toba , W. Masahashi , and T. Hashi-moto-Gotoh .
High-copy-number and low-copy-number plasmid vectors for lacZ a-complementation and chloramphen-icol or kanamycin selection .
Tano , K. , S. Shiota , J. Collier , R. S. Foote , and S. Mitra .
Isolation and structural characterization of a cDNA clone encoding the human DNA repair protein for 6-alkylguanine .
Proteolytic processing of the Ada protein that repairs DNA 06-methylguanine residues in E. coli .
Teo , I. , B. Sedgwick , B. Demple , B. Li , and T. Lindahl .
Induction of resistance to alkylating agents in E. coli : the ada + gene product serves both as a regulatory protein and as an enzyme for repair of mutagenic damage .
Teo , I. , B. Sedgwick , M. W. Kilpatrick , T. V. McCarthy , and T. Lindahl .
The intracellular signal for induction of resistance to alkylating agents in E. coli .
Vaughan , P. , and B. Sedgwick .
A weak adaptive response to alkylation damage in Salmonella typhimurium .
Adaptive response of Escherichia coli to alkylation damage .
Watanabe , M. , T. Nohmi , and M. Ishidate , Jr. 1987 .
New tester strains of Salmonella typhimurium highly sensitive to mutagenic nitroarenes .
Weinfeld , M. , A. F. Drake , J. K. Saunders , and M. C. Paterson .
Stereospecific removal of methyl phosphotriesters from DNA by an Escherichia coli ada + extract .
Wilkinson , M. C. , P. M. Potter , L. Cawkweli , P. Georgiadis , D. Patel , P. F. Swann , and G. P. Margison .
Purification of the E. coli ogt gene product to homogeneity and its rate of action on 06-methylguanine , 06-ethylguanine and 04-methylthymine in dodecadeoxyribonucleotides .