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Sgs1 Regulates Gene Conversion Tract Lengths and Crossovers Independently of Its Helicase Activity

Yi-Chen Lo, Kimberly S. Paffett, Or Amit, Jennifer A. Clikeman, Rosa Sterk, Mark A. Brenneman, and Jac A. Nickoloff^*

Department of Molecular Genetics and Microbiology, University of New Mexico School of Medicine, Cancer Research and Treatment Center, Albuquerque, New Mexico 87131

Received 23 January 2006/ Returned for modification 6 March 2006/ Accepted 16 March 2006

RecQ helicases maintain genome stability and suppress tumors in higher eukaryotes through roles in replication and DNA repair. The yeast RecQ homolog Sgs1 interacts with Top3 topoisomerase and Rmi1. In vitro, Sgs1 binds to and branch migrates Holliday junctions (HJs) and the human RecQ homolog BLM, with Top3, resolves synthetic double HJs in a noncrossover sense. Sgs1 suppresses crossovers during the homologous recombination (HR) repair of DNA double-strand breaks (DSBs). Crossovers are associated with long gene conversion tracts, suggesting a model in which Sgs1 helicase catalyzes reverse branch migration and convergence of double HJs for noncrossover resolution by Top3. Consistent with this model, we show that allelic crossovers and gene conversion tract lengths are increased in sgs1. However, crossover and tract length suppression was independent of Sgs1 helicase activity, which argues against helicase-dependent HJ convergence. HJs may converge passively by a "random walk," and Sgs1 may play a structural role in stimulating Top3-dependent resolution. In addition to the new helicase-independent functions for Sgs1 in crossover and tract length control, we define three new helicase-dependent functions, including the suppression of chromosome loss, chromosome missegregation, and synthetic lethality in srs2. We propose that Sgs1 has helicase-dependent functions in replication and helicase-independent functions in DSB repair by HR.

Supplemental material for this article may be found at http://mcb.asm.org/.

Present address: Department of Genetics, Rutgers University, Piscataway, NJ 08854.

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