1. Institute of Biotechnology and Department of Molecular Medicine, University of Texas Health Science Center at San Antonio, 15355 Lambda Drive, San Antonio, Texas 78245, USA
2. Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115, USA
3. Department of Biochemistry, New York University School of Medicine, New York, New York 10016, USA
4. These authors contributed equally to the work

Correspondence to: Lumir Krejci^1,2Patrick Sung¹ Correspondence and requests for materials should be addressed to L.K. (Email: krejci@uthscsa.edu) or P.S. (Email: sung@uthscsa.edu).

Abstract

Mutations in the Saccharomyces cerevisiae gene SRS2 result in the yeast's sensitivity to genotoxic agents, failure to recover or adapt from DNA damage checkpoint-mediated cell cycle arrest, slow growth, chromosome loss, and hyper-recombination^{1, 2}. Furthermore, double mutant strains, with mutations in DNA helicase genes SRS2 and SGS1, show low viability that can be overcome by inactivating recombination, implying that untimely recombination is the cause of growth impairment^{1, 3, 4}. Here we clarify the role of SRS2 in recombination modulation by purifying its encoded product and examining its interactions with the Rad51 recombinase. Srs2 has a robust ATPase activity that is dependent on single-stranded DNA (ssDNA) and binds Rad51, but the addition of a catalytic quantity of Srs2 to Rad51-mediated recombination reactions causes severe inhibition of these reactions. We show that Srs2 acts by dislodging Rad51 from ssDNA. Thus, the attenuation of recombination efficiency by Srs2 stems primarily from its ability to dismantle the Rad51 presynaptic filament efficiently. Our findings have implications for the basis of Bloom's and Werner's syndromes, which are caused by mutations in DNA helicases and are characterized by increased frequencies of recombination and a predisposition to cancers and accelerated ageing⁵.