Comparison of the PR mutant with the wild-type strain of Proteus mirabilisbrings insight into peroxide resistance factors and regulation of catalaseexpression

The peroxide resistant mutant (PR) of Proteus mirabilis was characterized b y an increased constitutive catalase activity concomitant with a large prod uction of specific mRNA. Survival toward hydrogen peroxide during exponenti al phase was increased by H2O2 pretreatment in the wild type but not in the mutant, although the catalase of both strains was not inducible under thes e conditions. In the mutant, besides catalase, over-produced proteins compr ised two different alkyl hydroperoxide reductase subunit C (AhpC) proteins and a protein homologous to the stationary phase transcription factor SspA of Escherichia coli. Conversely, the flagellin A (FlaA) of P. mirabilis was repressed in the PR mutant. Genomic DNA fragments of 2.9 kb carrying the c atalase gene (katA) together with the 5' and 3' flanking regions were isola ted from both strains and found to be identical. Upstream of katA, a Fur bo x-like sequence was found, but surprisingly, restricting iron in the cultur e medium caused a decrease in catalase production. The PR mutant presents s imilarities with other peroxide resistant mutants, but the regulation of ca talase biosynthesis in P. mirabilis seems somewhat different from other clo se species such as E. coli.