The plasmid pUM505 contains the operon that encodes proteins similar to error-prone repair DNA polymerase V. The gene appears to be truncated and its product is probably not functional. The gene, renamed , possesses an SOS box overlapped with a Sigma factor 70 type promoter; accordingly, transcriptional fusions revealed that the gene promoter is activated by mitomycin C. The predicted sequence of the UmuDpR protein displays 23 % identity with the SOS-response LexA repressor. The gene caused increased MMC sensitivity when transferred to the PAO1 strain. As expected, PAO1-derived knockout mutant PW6037 showed resistance to MMC; however, when the gene was transferred to PW6037, MMC resistance level was reduced. These data suggested that UmuDpR represses the expression of SOS genes, as LexA does. To test whether UmuDpR exerts regulatory functions, expression of PAO1 SOS genes was evaluated by reverse transcription quantitative PCR assays in the mutant with or without the pUC_umuD recombinant plasmid. Expression of , and genes increased 3.4–5.3 times in the mutant, relative to transcription of the corresponding genes in the strain, but decreased significantly in the / transformant. These results confirmed that the UmuDpR protein is a repressor of SOS genes controlled by LexA. Electrophoretic mobility shift assays, however, did not show binding of UmuDpR to 5′ regions of SOS genes, suggesting an indirect mechanism of regulation.


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