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An Escherichia coli mutant was isolated and shown to be polymyxin B resistant. Mapping and sequence analysis revealed a missense mutation at codon 53 within the pmrA (basR) gene that results in a G-to-V substitution. Fusions of promoters from the pmrC, yibD, and pmrH genes with the lacZ reporter showed that they were constitutively expressed in pmrA53 cells. In pmr[A.sup.+] strains, these promoters were induced by iron and zinc, while a [DELTA]pmrA mutation blocked induction. The PmrA regulon regulates genes whose products remodel the composition and charge of lipid A and hence the barrier properties of the outer membrane. Along these lines, the pmrA53 mutant was also found to be hypersensitive to the anionic bile detergent deoxycholic acid.

The DNA-binding protein Fis binds to a region that overlaps the -35 and spacer sequences of the dnaAP2 promoter. This was gleaned from a band shift assay conducted to examine the ability of Fis to bind to the dnaA promoter region. The location of its binding site indicates that Fis is a repressor of the dnaAP2 promoter. This is affirmed by the observation that in a Fis mutant, as against a Fis wild-type strain, there is a twofold increase in the activity of the beta-galactosidase fusion protein expressed from the dnaA regulatory region.