Induction of UV-resistant DNA replication in Escherichia coli: induced stable DNA replication as an SOS function

The striking similarity between the treatments that induce SOS functions and those that result in stable DNA replication (continuous DNA replication in the absence of protein synthesis) prompted us to examine the possibility of stable DNA replication being a recA+ lexA+-dependent SOS function. In addition to the treatments previously reported, ultraviolet (UV) irradiation or treatment with mitomycin C was also found to induce stable DNA replication. The thermal treatment of tif-1 strains did not result in detectable levels of stable DNA replication, but nalidixic acid readily induced the activity in these strains. The induction of stable DNA replication with malidixic acid was severely suppressed in tif-1 lexA mutant strains. The inhibitory activity of lexA3 was negated by the presence of the spr-51 mutation, an intragenic suppressor of lexA3. Induced stable DNA replication was found to be considerably more resistant to UV irradiation than normal replication both in a uvrA6 strain and a uvr+ strain. The UV-resistant replication occurred mostly in the semiconservative manner. The possible roles of stable DNA replication in repair of damaged DNA are discussed.