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

Double mutants of Escherichia coli dam (DNA adenine methyltransferase) stra ins with ruvA, ruvB, or ruvC could not be constructed, whereas dam derivati ves with recD, recF, recJ, and recR were viable. The ruv gene products are required for Holliday junction translocation and resolution of recombinatio n intermediates. A dam recG (Holliday junction translocation) mutant strain was isolated but at a very much lower frequency than expected, The inviabi lity of a dam lexA (Ind(-)) host was abrogated by the simultaneous presence of plasmids encoding both recA and ruvAB. This result indicates that of mo re than 20 SOS genes, only recA and ruvAB need to be derepressed to allow f or dam mutant survival. The presence of mutS or mutL mutations allowed the construction of dam lex-A (Ind(-)) derivatives. The requirement for recA, r ecB, recC, ruvA, ruvB, ruvC, and possibly recG gene expression indicates th at recombination is essential for viability of dam bacteria probably to rep air DNA double-strand breaks, The effect of mutS and mutL mutations indicat es that DNA mismatch repair is the ultimate source of most of these DNA bre aks. The requirement for recombination also suggests an explanation for the sensitivity of darn cells to certain DNA-damaging agents.