This behavior is due to the highly divergent PmrD protein ( Figs. 6 and 7 ) , because replacement of the E. coli pmrD gene by the Salmonella pmrD gene endowed E. coli with the ability to transcribe the PmrA-activated pbgP gene and to exhibit resistance to polymyxin B in response to the low Mg2 signal ( Fig. 3 ) .
This phenotype was reflected also at the level of transcription ; the PmrA-activated pbgP gene was expressed by E. coli cells experiencing Fe3 but not in response to the low Mg2 signal ( Fig. 2C ) , whereas a pbgP transcript was made by Salmo-nella in both conditions ( Fig. 2D ) .
Although a non-cytotoxic form of polymyxin B -- termed polymyxin B nonapeptide ( Vaara and Vaara , 1983 ) -- could partially protect a Salmonella pmrA mutant from Fe ( III ) - mediated killing ( Chamnongpol et al. , 2002 ) inactivation of the PmrA-activated loci responsible for the lipid-A modification with 4-aminoarabinose ( i.e. pbgP ) or phosphoethanolamine ( i.e. pmrC ) did not render the organism susceptible to Fe ( III ) ( Lee et al. , 2004 ) .
DpbgE2 DpbgE3 pbgP Ugd DpbgE2/vector DpbgE2/ppbgE2 DpbgE3/vector DpbgE3/ppbgE3 pbgPDpbgE2 pbgPDpbgE3 ugdDpbgE2 ugdDpbgE3 DpmrAB/vector DpmrAB/ppmrAB DpmrAB/vector DpmrAB/ppbgE2 DpmrAB/ppbgE3 DpmrAB/ppbgE2E3 DpmrG DpmrC DpbgPE Dugd DyibD DpbgPE pmrC ugd pmrG yibD DpmrC ugd pmrG yibD DpmrC ugd DpmrC pmrG DpmrC yibD DyibD ugd DyibD pmrG Dugd pmrG DpmrC ugd yibD DpmrC pmrG yibD Dugd pmrG yibD DpmrC ugd pmrG pmrA505DpmrC ugd pmrG DpmrC ugd pmrG/vector DpmrC ugd pmrG/ppmrC DpmrC ugd pmrG/pugd DpmrC ugd pmrG/ppmrG DpmrG pbgPE DpmrC pbgPE DpmrC pbgPE pmrG The PmrA-activated ugd , pbgP , pmrC and pmrG genes are required for Fe ( III ) resistance To examine whether the pmrG and pmrC genes are necessary for Fe ( III ) resistance , we constructed strains deleted for the chromosomal copies of these genes ( see Experimental procedures ) .
DpbgE2 DpbgE3 pbgP Ugd DpbgE2/vector DpbgE2/ppbgE2 DpbgE3/vector DpbgE3/ppbgE3 pbgPDpbgE2 pbgPDpbgE3 ugdDpbgE2 ugdDpbgE3 DpmrAB/vector DpmrAB/ppmrAB DpmrAB/vector DpmrAB/ppbgE2 DpmrAB/ppbgE3 DpmrAB/ppbgE2E3 DpmrG DpmrC DpbgPE Dugd DyibD DpbgPE pmrC ugd pmrG yibD DpmrC ugd pmrG yibD DpmrC ugd DpmrC pmrG DpmrC yibD DyibD ugd DyibD pmrG Dugd pmrG DpmrC ugd yibD DpmrC pmrG yibD Dugd pmrG yibD DpmrC ugd pmrG pmrA505DpmrC ugd pmrG DpmrC ugd pmrG/vector DpmrC ugd pmrG/ppmrC DpmrC ugd pmrG/pugd DpmrC ugd pmrG/ppmrG DpmrG pbgPE DpmrC pbgPE DpmrC pbgPE pmrG The PmrA-activated ugd , pbgP , pmrC and pmrG genes are required for Fe ( III ) resistance To examine whether the pmrG and pmrC genes are necessary for Fe ( III ) resistance , we constructed strains deleted for the chromosomal copies of these genes ( see Experimental procedures ) .
For example , activation of the PmrA/PmrB system ( 26 , 27 ) by growing Salmonella in media containing a high ( 10 mM ) Mg2 + concentration and then shifting it to media containing a low 2 + 3 + ( 50 mM ) Mg concentration and 100 mM Fe resulted in an increase in the mRNA levels corresponding to the PmrA-activated pbgP and pmrC genes , which peaked and then reached new steady-state levels ( fig .
Synthesis of Ara4N and addition to the lipid-A is mediated by the Salmonella PmrA-activated genes pmrE ( also known as ugd ) and pmrHFIJKLM ( the pmrH operon , also known as the pbgP or arn operon ) ( Gunn et al. , 2000 ) .
To test this hypothesis , we determined the β2 galactosidase activity produced by strains harboring chromosomal lac transcriptional-fusions to the PmrA-activated pbgP operon and ugd gene .