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Vol. 17, Issue 4, 2081-2090, April 2006

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Rad22^Rad52-dependent Repair of Ribosomal DNA Repeats Cleaved by Slx1-Slx4 Endonuclease

Stéphane Coulon ^*, Eishi Noguchi , Chiaki Noguchi , Li-Lin Du ^*, Toru M. Nakamura ^* , and Paul Russell ^*

^* Department of Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037; Department of Biochemistry and Molecular Biology, Drexel University College of Medicine, Philadelphia, PA 19102

Submitted November 2, 2005; Revised January 26, 2006; Accepted January 30, 2006
Monitoring Editor: Orna Cohen-Fix

Slx1 and Slx4 are subunits of a structure-specific DNA endonuclease that is found in Saccharomyces cerevisiae, Schizosaccharomyces pombe, and other eukaryotic species. It is thought to initiate recombination events or process recombination structures that occur during the replication of the tandem repeats of the ribosomal DNA (rDNA) locus. Here, we present evidence that fission yeast Slx1-Slx4 initiates homologous recombination events in the rDNA repeats that are processed by a mechanism that requires Rad22 (Rad52 homologue) but not Rhp51 (Rad51 homologue). Slx1 is required to generate 50% of the spontaneous Rad22 DNA repair foci that occur in cycling cells. Most of these foci colocalize with the nucleolus, which contains the rDNA repeats. The increased fork pausing at the replication fork barriers in the rDNA repeats in a strain that lacks Rqh1 DNA helicase is further increased by expression of a dominant negative form of Slx1. These data suggest that Slx1-Slx4 cleaves paused replication forks in the rDNA, leading to Rad22-dependent homologous recombination that is used to maintain rDNA copy number.

Abbreviations used: ARS, autonomously replicating sequence; 2D, two-dimensional; BIR, break-induced replication; DAPI, 4,6-diamidino-2-phenylindole; DSB, double-strand break; HJ, Holliday-Junction; HU, hydroxyurea; MMS, methyl methanesulfonate; RFB, replication fork barrier; WCE, whole cell extract.

Present address: Department of Biochemistry and Molecular Genetics, University of Illinois at Chicago, Chicago, IL 60607.

Address correspondence to: Paul Russell (prussell{at}scripps.edu).

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