UvrD303, a hyperhelicase mutant that antagonizes RecA-dependent SOS expression by a mechanism that depends on its C terminus.

Genomic integrity is critical for an organism's survival and ability to reproduce. In Escherichia coli, the UvrD helicase has roles in nucleotide excision repair and methyl-directed mismatch repair and can limit reactions by RecA under certain circumstances. UvrD303 (D403A D404A) is a hyperhelicase mutant, and when expressed from a multicopy plasmid, it results in UV sensitivity (UV(s)), recombination deficiency, and antimutability. In order to understand the molecular mechanism underlying the UV(s) phenotype of uvrD303 cells, this mutation was transferred to the E. coli chromosome and studied in single copy. It is shown here that uvrD303 mutants are UV sensitive, recombination deficient, and antimutable and additionally have a moderate defect in inducing the SOS response after UV treatment. The UV-sensitive phenotype is epistatic with recA and additive with uvrA and is partially suppressed by removing the LexA repressor. Furthermore, uvrD303 is able to inhibit constitutive SOS expression caused by the recA730 mutation. The ability of UvrD303 to antagonize SOS expression was dependent on its 40 C-terminal amino acids. It is proposed that UvrD303, via its C terminus, can decrease the levels of RecA activity in the cell.