Perturbed chromosomal replication in recA mutants of Escherichia coli.

When initiation of DNA replication is inhibited in wild-type Escherichia coli cells by rifampin or chloramphenicol, completion of ongoing rounds of replication (runout of replication) leads to cells containing two, four, or eight fully replicated chromosomes, as measured by flow cytometry. In recombination-deficient recA strains, a high frequency of cells with three, five, six, or seven fully replicated chromosomes was observed in addition to cells with two, four, or eight chromosomes. recA mutants affected only in the protease-stimulating function behaved like wild-type cells. Thus, in the absence of the recombinase function of RecA protein, the frequency of productive initiations was significantly reduced compared with that in its presence. DNA degradation during runout of replication in the presence of rifampin was about 15%. The DNA degradation necessary to account for the whole effect described above was in this range or even lower. However, a model involving selective and complete degradation of partially replicated chromosomes is considered unlikely. It is suggested that the lack of RecA protein causes initiations or newly formed replication forks to stall but remain reactivatable for a period of time by functional RecA protein.