Identification of additional genes belonging to the LexA regulon in Escherichia coli

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 gene s in the SOS regulon are controlled by the LexA protein. LexA acts as a tra nscriptional repressor of these unlinked genes by binding to specific seque nces (LexA boxes) located within the promoter region of each LexA-regulated gene. Alignment of 20 LexA binding sites found in the E. coli chromosome r eveals a consensus of 5'-TACTG(TA)(5)CAGTA-3'. DNA sequences that exhibit a close match to the consensus are said to have a low heterology index and b ind LexA tightly, whereas those that are more diverged have a high heterolo gy 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 comp utational searches of the entire E. coli genome to identify novel LexA-regu lated genes. These searches identified a total of 69 potential LexA-regulat ed genes/operons with a heterology index of < 15 and included all previousl y characterized LexA-regulated genes. Probes were made to the remaining gen es, and these were screened by Northern analysis for damage-inducible gene expression in a wild-type lexA(+) cell, constitutive expression in a lexA(D ef) cell and basal expression in a non-inducible lexA(Ind(-)) cell. These e xperiments 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. Th e potential function of each newly identified LexA-regulated gene is discus sed.