This result is surprising because : ( i ) E. coli encodes homologues of the PhoP PhoQ and PmrA PmrB systems , the PmrD protein , as well as the PmrA-regulated genes mediating the LPS modifications required for polymyxin B resistance ( 24 ) ; ( ii ) its PhoP PhoQ system also responds to Mg2 ( 25 ) ; and ( iii ) mutations in its pmrA gene produce strains with the same phenotypes as Salmo-nella pmrA mutants ( 8 , 12 ) .