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Elife. 2017 May 23;6. pii: e22195. doi: 10.7554/eLife.22195.

Srs2 promotes synthesis-dependent strand annealing by disrupting DNA polymerase δ-extending D-loops.

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Department of Microbiology and Molecular Genetics, University of California, Davis, Davis, United States.
Department of Biochemistry, University of Iowa Carver College of Medicine, Iowa City, United States.
DRF-IRCM-CIGEx, CEA, Fontenay aux Roses, France.
Department of Molecular and Cellular Biology, University of California, Davis, Davis, United States.


Synthesis-dependent strand annealing (SDSA) is the preferred mode of homologous recombination in somatic cells leading to an obligatory non-crossover outcome, thus avoiding the potential for chromosomal rearrangements and loss of heterozygosity. Genetic analysis identified the Srs2 helicase as a prime candidate to promote SDSA. Here, we demonstrate that Srs2 disrupts D-loops in an ATP-dependent fashion and with a distinct polarity. Specifically, we partly reconstitute the SDSA pathway using Rad51, Rad54, RPA, RFC, DNA Polymerase δ with different forms of PCNA. Consistent with genetic data showing the requirement for SUMO and PCNA binding for the SDSA role of Srs2, Srs2 displays a slight but significant preference to disrupt extending D-loops over unextended D-loops when SUMOylated PCNA is present, compared to unmodified PCNA or monoubiquitinated PCNA. Our data establish a biochemical mechanism for the role of Srs2 in crossover suppression by promoting SDSA through disruption of extended D-loops.


DNA helicase; S. cerevisiae; biochemistry; chromosomes; crossover avoidance; genes; genome stability; homologous rcombination

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