Escherichia coli single-strand DNA binding protein from wild type and lexC113 mutant affects in vitro proteolytic cleavage of phage lambda repressor.

In Escherichia coli, the single-strand DNA-binding protein (SSB) is required for DNA replication. A mutation of the ssb gene, lexC113, imparts to the cells UV sensitivity and inability to induce lambda prophage and to amplify recA protein, indicating participation of SSB in DNA repair and viral induction processes. We report the effect of purified SSB, isolated from wild-type and lexC113 strains, on the recA-mediated proteolysis of lambda repressor in vitro. (i) These proteins abolished the inhibition produced by excess single-strand DNA and (ii) in the presence of the binding proteins, the apparent stoichiometry--1 monomer of recA to 6 nucleotides of single-strand DNA [Craig, N. L. & Roberts, J. W. (1980) Nature (London) 283, 26-30] was no longer observed. (iii) At the optimal concentration--1 protein monomer to 8 nucleotides--they increased the rate and extent of repressor cleavage at all single-strand DNA concentrations, including that observed at the apparent optimal DNA concentration. (iv) At binding protein/nucleotide ratios greater than or equal to 1:3, SSB from lexC113 inhibited repressor cleavage while that from wild type did not. (v) These results are consistent with the notion that SSB is probably involved in the induction of prophages in vivo.