UvrA and UvrB suppress illegitimate recombination: Synergistic action with RecQ helicase

Illegitimate recombination is a major cause of genetic instability in prokaryotes as well as in eukaryotes. This recombination usually occurs at a low frequency, but it is greatly enhanced by UV irradiation or other environmental stresses. DNA damages produced by these environmental stresses are thought to induce DNA double-strand breaks, leading to illegitimate recombination. In this paper we show that UV-induced illegitimate recombination is enhanced by mutations of nucleotide excision repair genes, uvrA or uvrB, and partially by uvrC mutation, but not by uvrD mutation. Unexpectedly, the recombination was enhanced by the uvrA uvrB double mutation even without UV irradiation, but the uvrB uvrC double mutation has not shown this effect, suggesting that illegitimate recombination is mostly suppressed by UvrA and UvrB. Moreover, illegitimate recombination was synergistically enhanced by the recQ uvrA double mutation. In addition, overproduction of the UvrA protein suppressed the hyperrecombination phenotype of the recQ or uvrB mutant, but it did not affect the UV-sensitive phenotype of the uvrB mutant. We concluded that the UvrAB complex suppresses illegitimate recombination in a pathway shared with RecQ helicase. In addition, UvrA protein alone can suppress illegitimate recombination in the pathway, in which RecQ helicase and UvrAB complex work. Possible functions of the proteins involved in these pathways are also discussed.