Rad52 sumoylation prevents the toxicity of unproductive Rad51 filaments independently of the anti-recombinase Srs2

The budding yeast Srs2 is the archetype of helicases that regulate several aspects of homologous recombination (HR) to maintain genomic stability. Srs2 inhibits HR at replication forks and prevents high frequencies of crossing-over. Additionally, sensitivity to DNA damage and synthetic lethality with replication and recombination mutants are phenotypes that can only be attributed to another role of Srs2: the elimination of lethal intermediates formed by recombination proteins. To shed light on these intermediates, we searched for mutations that bypass the requirement of Srs2 in DNA repair without affecting HR. Remarkably, we isolated rad52-L264P, a novel allele of RAD52, a gene that encodes one of the most central recombination proteins in yeast. This mutation suppresses a broad spectrum of srs2Δ phenotypes in haploid cells, such as UV and γ-ray sensitivities as well as synthetic lethality with replication and recombination mutants, while it does not significantly affect Rad52 functions in HR and DNA repair. Extensive analysis of the genetic interactions between rad52-L264P and srs2Δ shows that rad52-L264P bypasses the requirement for Srs2 specifically for the prevention of toxic Rad51 filaments. Conversely, this Rad52 mutant cannot restore viability of srs2Δ cells that accumulate intertwined recombination intermediates which are normally processed by Srs2 post-synaptic functions. The avoidance of toxic Rad51 filaments by Rad52-L264P can be explained by a modification of its Rad51 filament mediator activity, as indicated by Chromatin immunoprecipitation and biochemical analysis. Remarkably, sensitivity to DNA damage of srs2Δ cells can also be overcome by stimulating Rad52 sumoylation through overexpression of the sumo-ligase SIZ2, or by replacing Rad52 by a Rad52-SUMO fusion protein. We propose that, like the rad52-L264P mutation, sumoylation modifies Rad52 activity thereby changing the properties of Rad51 filaments. This conclusion is strengthened by the finding that Rad52 is often associated with complete Rad51 filaments in vitro.

Homologous recombination (HR) is essential for double-strand break repair and participates in post-replication restart of stalled and collapsed replication forks. However, HR can lead to genome rearrangements and has to be strictly controlled. The budding yeast Srs2 is involved in the prevention of high crossing-over frequencies and in the inhibition of HR at replication forks. Nevertheless, important phenotypes of srs2Δ mutants, like sensitivity to DNA damage and synthetic lethality with replication and recombination mutants, can only be attributed to another role of Srs2: the elimination of lethal intermediates formed by recombination proteins. The nature of these intermediates remains to be defined. In a screen designed to uncover mutations able to suppress srs2Δ phenotypes, we isolated a novel allele of Rad52 (rad52-L264P), the gene that codes for the major Rad51 nucleoprotein filament mediator. Interestingly, we observed that rad52-L264P bypasses the requirement for Srs2 without affecting DNA repair by HR. We also found that Rad52-L264P specifically prevents the formation of unproductive Rad51 filaments before strand invasion, allowing us to define Srs2 substrates. Further analysis showed that Rad52-L264P mimics the properties of the Rad52-SUMO conjugate, revealing that Rad52 assembles Rad51 filaments differently according to its sumoylation status.