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 ϳ50% 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.
INTRODUCTIONWhile replicating the genome, DNA polymerases will sometimes stall when they encounter DNA lesions, protein complexes bound to DNA, or nucleotide starvation. Stalled forks pose serious threats to genomic integrity because they are prone to collapse or rearrangement (McGlynn and Lloyd, 2002). In contrast, there are many examples of natural replication pause and termination sites in a variety of organisms (Rothstein et al., 2000). These sites are often called replication fork barriers (RFBs). It is somewhat paradoxical that in some cases these "programmed" RFBs are thought to play important roles in maintaining genome integrity.The best-characterized RFBs occur in the ribosomal DNA genes (rDNA), which are present in long tandem repeats at one or a few chromosomal loci in most eukaryotic species, including yeasts and humans (Brewer and Fangman, 1988;Linskens and Huberman, 1988;Little et al., 1993;Gerber et al., 1997;Sanchez et al., 1998). The budding yeast Saccharomyces cerevisiae carries ϳ150 copies of the rDNA genes. The RFB is located in the nontranscribed sequence (NTS) between the 3Ј end of the 35S rRNA gene and the autonomously replicating sequence (ARS). The RFB inhibits replication fork progression in the direction opposite to rDNA transcription. This mechanism helps to prevent collisions between replication forks and the transcription machinery (Takeuchi et al., 2003). The protein fork blocking 1 (Fob1) binds DNA in the RFB region and is essential for RFB activity (Kobayashi and Horiuchi, 1996).The size of the rDNA array in S. cerevisiae is not static. For example, it will contract upon inactivation of RNA polymerase I and expands after RNA polymerase I activity is restored (Kobayashi et al., 1998). Fob1 is required for the mechanisms that control the copy number of rDNA repeats (Kobayashi et al., 2001;Johzuka and Horiuchi, 2002). The current model for the regulation of the rDNA copy number involv...