The Srs2 helicase prevents recombination by disrupting Rad51 nucleoprotein filaments

Abstract

Homologous recombination is a ubiquitous process with key functions in meiotic and vegetative cells for the repair of DNA breaks. It is initiated by the formation of single-stranded DNA on which recombination proteins bind to form a nucleoprotein filament that is active in searching for homology, in the formation of joint molecules and in the exchange of DNA strands. This process contributes to genome stability but it is also potentially dangerous to cells if intermediates are formed that cannot be processed normally and thus are toxic or generate genomic rearrangements. Cells must therefore have developed strategies to survey recombination and to prevent the occurrence of such deleterious events. In Saccharomyces cerevisiae, genetic data have shown that the Srs2 helicase negatively modulates recombination, and later experiments suggested that it reverses intermediate recombination structures. Here we show that DNA strand exchange mediated in vitro by Rad51 is inhibited by Srs2, and that Srs2 disrupts Rad51 filaments formed on single-stranded DNA. These data provide an explanation for the anti-recombinogenic role of Srs2 in vivo and highlight a previously unknown mechanism for recombination control.

Genetic evidence for a role of Saccharomyces cerevisiae Mph1 in recombinational DNA repair under replicative stress

“…Thus, the 3 -end of the leading strand is elongated beyond the lesion on the lagging strand as template. Srs2 could discourage D-loop formation acting at a presynaptic stage (Krejci et al, 2003;Veaute et al, 2003) or a postsynaptic stage by disrupting the D-loop to relocate the formerly arrested, now elongated, strand back onto its original template. This is similar to its function in promoting SDSA in double-strand break repair ( Figure 5E; Ira et al, 2003;Dupaigne et al, 2008).…”

“…Monoubiquitinated PCNA can be polyubiquitinated by the Rad5-Ubc13-Mms2 complex (Hofmann and Pickart, 1999) to allow error-free mechanisms (Torres-Ramos et al, 2002). Alternatively, K164 of PCNA can be SUMOylated to regulate recombination at the fork (Pfander et al, 2005;Moldovan et al, 2006) via a direct interaction with Srs2 (Pfander et al, 2005), a 3 -5 DNA helicase (Rong and Klein, 1993), thought to exert a negative control on HR (Krejci et al, 2003;Veaute et al, 2003).…”

Eukaryotic DNA damage tolerance and translesion synthesis through covalent modifications of PCNA

“…Genetic data indicate that Srs2 negatively regulates recombination [70,71] possibly by reversal of intermediate recombination structures [72][73][74][75]. Indeed, the DNA strand exchange mediated by Rad51 is inhibited by Srs2 through disruption of the Rad51-ssDNA filaments [76,77], and it turns out that sumoylated PCNA has increased affinity for Srs2 [78,79] and represses the Rad52-dependent recombination pathway [80]. These observations collectively support the hypothesis that Srs2 serves as a molecular switch between homologous recombination and DDT [6], and further confirm that the sensor for this switch is the state of PCNA modification.…”

Multiple Genetic Pathways Involving the Caenorhabditis elegans Bloom's Syndrome Genes him-6, rad-51, and top-3 Are Needed To Maintain Genome Stability in the Germ Line

“…The phenotype of sgs1 mutants includes a slight reduction in growth rate, a 10-fold increase in chromosome missegregation, an elevated mitotic recombination frequency, a high sensitivity to methyl methanesulfonate and hydroxyurea (HU) (15,66), and premature aging as a consequence of an increased number of extrachromosomal ribosomal DNA circles in the cell (17,51,60,61). Sgs1p acts redundantly with the budding yeast helicase Srs2p, which has been shown to inhibit loading of Rad51p onto single-stranded DNA (ssDNA) in vitro (33,56). In addition, the sgs1 mutants display some meiotic defects such as delayed meiotic prophase and defective chromosome synapsis (16,45).…”