Exposure of Escherichia coli to a variety of DNA‐damaging agents results in the induction of the global ‘SOS response’. Expression of many of the genes in the SOS regulon are controlled by the LexA protein. LexA acts as a transcriptional repressor of these unlinked genes by binding to specific sequences (LexA boxes) located within the promoter region of each LexA‐regulated gene. Alignment of 20 LexA binding sites found in the E. coli chromosome reveals a consensus of 5′‐TACTG(TA)₅CAGTA‐3′. DNA sequences that exhibit a close match to the consensus are said to have a low heterology index and bind LexA tightly, whereas those that are more diverged have a high heterology index and are not expected to bind LexA. By using this heterology index, together with other search criteria, such as the location of the putative LexA box relative to a gene or to promoter elements, we have performed computational searches of the entire E. coli genome to identify novel LexA‐regulated genes. These searches identified a total of 69 potential LexA‐regulated genes/operons with a heterology index of < 15 and included all previously characterized LexA‐regulated genes. Probes were made to the remaining genes, and these were screened by Northern analysis for damage‐inducible gene expression in a wild‐type lexA⁺ cell, constitutive expression in a lexA(Def) cell and basal expression in a non‐inducible lexA(Ind⁻) cell. These experiments have allowed us to identify seven new LexA‐regulated genes, thus bringing the present number of genes in the E. coli LexA regulon to 31. The potential function of each newly identified LexA‐regulated gene is discussed.

The damage-inducible UmuD′ and UmuC proteins are required for most SOS mutagenesis in Escherichia coli . Our recent assay to reconstitute this process in vitro , using a native UmuD′ ₂ C complex, revealed that the highly purified preparation contained DNA polymerase activity. Here we eliminate the possibility that this activity is caused by a contaminating DNA polymerase and show that it is intrinsic to UmuD′ ₂ C. E. coli dinB has recently been shown to have DNA polymerase activity (pol IV). We suggest that UmuD′ ₂ C, the fifth DNA polymerase discovered in E. coli , be designated as E. coli pol V. In the presence of RecA, β sliding clamp, γ clamp loading complex, and E. coli single-stranded binding protein (SSB), pol V’s polymerase activity is highly “error prone” at both damaged and undamaged DNA template sites, catalyzing efficient bypass of abasic lesions that would otherwise severely inhibit replication by pol III holoenzyme complex (HE). Pol V bypasses a site-directed abasic lesion with an efficiency about 100- to 150-fold higher than pol III HE. In accordance with the “A-rule,” dAMP is preferentially incorporated opposite the lesion. A pol V mutant, UmuD′ ₂ C104 (D101N), has no measurable lesion bypass activity. A kinetic analysis shows that addition of increasing amounts of pol III to a fixed level of pol V inhibits lesion bypass, demonstrating that both enzymes compete for free 3′-OH template-primer ends. We show, however, that despite competition for primer-3′-ends, pol V and pol III HE can nevertheless interact synergistically to stimulate synthesis downstream from a template lesion.