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Mol Cell. 2008 Feb 1;29(2):243-54. doi: 10.1016/j.molcel.2007.11.033.

The Srs2 helicase activity is stimulated by Rad51 filaments on dsDNA: implications for crossover incidence during mitotic recombination.

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Laboratorie de Microscopie Moléculaire et Cellulaire, UMR 8126, Interactions Moléculaires et Cancer CNRS-Université Paris Sud-Institut de Cancérologie Gustave Roussy, Villejuif, France.


Saccharomyces cerevisiae Srs2 helicase was shown to displace Rad51 in vitro upon translocation on single-stranded DNA. This activity is sufficient to account for its antirecombination effect and for the elimination of otherwise dead-end recombination intermediates. Roles for the helicase activity are yet unknown. Because cells lacking Srs2 show increased incidence of mitotic crossovers, it was postulated that Srs2 promotes synthesis-dependent strand annealing (SDSA) by unwinding the elongating invading strand from the donor strand. We report here that synthetic DNA structures that mimic D loops are good substrates for the Srs2 helicase activity, that Srs2 translocates on RPA-coated ssDNA, and, furthermore, that the helicase activity is largely stimulated by the presence of Rad51 nucleoprotein filaments on double-stranded DNA. These properties strongly support the idea that Srs2 actively prevents crossovers by promoting SDSA.

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