Recombination is essential for viability of an Escherichia coli dam (DNA adenine methyltransferase) mutant.

expected. The inviability of a dam lexA (Ind 2 ) host was abrogated by the simultaneous presence of plasmids encoding both recA and ruvAB. This result indicates that of more than 20 SOS genes, only recA and ruvAB need to be derepressed to allow for dam mutant survival. The presence of mutS or mutL mutations allowed the construction of dam lexA (Ind 2 ) derivatives. The requirement for recA, recB, recC, ruvA, ruvB, ruvC, and possibly recG gene expression indicates that recombination is essential for viability of dam bacteria probably to repair DNA double-strand breaks. The effect of mutS and mutL mutations indicates that DNA mismatch repair is the ultimate source of most of these DNA breaks. The requirement for recombination also suggests an explanation for the sensitivity of dam cells to certain DNA-damaging agents. The dam gene of Escherichia coli encodes a DNA methyltransferase that methylates adenine in -GATC- sequences in double-stranded DNA (17). Mutant strains lacking this enzyme display a pleiotropic phenotype including increased mutability, hyperrecombination, and increased sensitivity to DNA-damaging agents. In addition, dam bacteria have an increased number of single-strand breaks in DNA compared to wild type. The phenotypes displayed by dam mutants are consistent with multiple roles of unmethylated, methylated, and hemimethylated -GATC- sequences in cellular physiology. These include regulation of gene expression and strand discrimination during replication-associated DNA mismatch repair (17). An additional feature of dam strains is inviability when combined with mutant alleles of recA, recB, recC, or noninducible (Ind 2 ) lexA (19). The lexA inviability suggests a requirement for derepression of one or more SOS genes. The SOS response is induced following treatments that damage DNA or inhibit DNA replication (6). About 20 genes (including recA, lexA, and ruvAB) that are negatively regulated by LexA are derepressed following cleavage of the LexA repressor. Treatments that induce the SOS regulon do so by activating the coprotease activity of RecA (“activated RecA”), resulting in LexA cleavage. RecA protein also catalyzes 39-single-strand invasion of homologous DNA and is, therefore, essential in the recombination process (15). Peterson et al. (24) showed that dam bacteria with a temperature-sensitive lexA allele were viable at 42°C but not at 30°C, indicating the requirement for derepressed expression of one or more LexA-regulated SOS genes. In addition, Peterson et al. (24) found higher basal-level expression (two- to sixfold) of several SOS genes (including recA, lexA, sulA, uvrA, uvrB, uvrD, dinD, and recF )i ndam mutants than in wild type. However, since other genes are also induced by LexA cleavage, it was not possible to determine which are required for dam viability. In the present communication, the SOS genes required for viability of dam strains have been identified. They are recA and ruvAB, the latter encoding enzymes that translocate Holliday junctions (15, 29). Two other non-SOS genes have also been identified. The recG gene product can also catalyze translocation (15), and dam recG mutants are probably inviable. It is also shown that expression of the ruvC gene product, a Holliday junction resolvase (15, 29), is also required for dam mutant viability. The requirement for recA, recB, recC, recG, ruvA, ruvB, and ruvC gene expression indicates that recombination is essential for dam mutant viability.