Effects of overproduction of single-stranded DNA-binding protein on RecA protein-dependent processes in Escherichia coli.

Overproduction of single-stranded DNA-binding protein (SSB) in Escherichia coli led to a decrease in the basal level of repressor LexA. Expression of the LexA-controlled genes was increased differentially, depending on the affinity of the LexA repressor for each promoter: expression of the recA and sfiA genes was increased 5-fold and 1.5-fold, respectively. Despite only a slight effect on expression of sfiA, which codes for an inhibitor of cell division, bacteria overproducing SSB produced elongated cells. In fact, the effect on cell shape appeared to be essentially independent of the expression of the sfiA and recA genes. Bacteria overproducing SSB were therefore phenotypically similar to bacteria partially starved of thymine, in which filamentation results from both sfiA-dependent and sfiA-recA-independent pathways. These data indicate that excess SSB acts primarily by perturbing DNA replication, thereby favoring gratuitous activation of RecA protein to promote cleavage of LexA protein. When bacteria overproducing SSB were exposed to a DNA-damaging agent such as ultraviolet light or mitomycin C, the recA and sfiA genes were fully induced. Induction of the sfiA gene occurred, however, at higher doses in bacteria overproducing SSB protein than in bacteria with normal levels of SSB. Whereas the efficiency of excision repair was apparently increased by excess SSB, the efficiency of post-replication recombinational repair was reduced as judged by a decrease in the recombination proficiency between a prophage and ultraviolet-irradiated heteroimmune infecting phage. Following induction of ssb+ bacteria with mitomycin C, the cellular content of SSB was slightly increased. These results provide evidence that SSB modulates RecA protein-dependent activities in vivo. It is proposed that SSB favors the formation of short complexes of RecA protein and single-stranded DNA that mediate cleavage of the LexA and lambda repressors, while it delays the formation of long nucleoprotein filaments, thereby slowing down RecA-promoted recombinational events in uninduced as well as in induced bacteria.