Influence of single-stranded DNA-binding protein on recA induction in Escherichia coli

Two ssb mutants of Escherichia coli, which carry a lesion in the single-strand DNA-binding protein (SSB), are sensitive to UV-irradiation. We have investigated the influence of SSB on the "SOS" repair pathway by examining the levels of recA protein synthesis. These strains fail to induce normal levels of recA protein after treatment with nalidixic acid or ultraviolet light. The level of recA protein synthesis in wild-type cells is about three times greater than ssb cells. This deficiency in ssb mutants occurs in all strains and at all temperatures tested (30-41.5 degrees). In contrast, the ssb-1 mutation has no effect on temperature-induced recA induction in a recA441 (tif-1) strain. Cells carrying ssb+ plasmids and overproducing normal DNA-binding protein surprisingly are moderately UV-sensitive and have reduced levels of recA protein synthesis. Together these results establish that single-strand DNA-binding protein is involved in the induction of recA, and accounts, at least in part, for the UV sensitivity of ssb mutants. Three possible mechanisms to explain the role of SSB are discussed.