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

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Abstract
In E. 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. The effect of purified SSB, isolated from wild-type and lexC113 strains, on the recA-mediated proteolysis of .lambda. repressor in vitro was studied. These proteins abolished the inhibition produced by excess single-strand DNA and in the presence of the binding proteins, the apparent stoichiometry-1 monomer of recA to 6 nucleotides of single-strand DNA was no longer observed. At the optimal concentration-1 protein monomer to 8 nucleotides-the proteins increased the rate and extent of repressor cleavage at all single-strand DNA concentrations, including that observed at the apparent optimal DNA concentration. At binding protein/nucleotide ratios .gtoreq. 1:3, SSB from lexC113 inhibited repressor cleavage while that from wild type did not. SSB is probably involved in the induction of prophages in vivo.