In the yeast Saccharomyces cerevisiae, meiotic recombination is initiated by DNA double-stranded breaks (DSBs) occurring in micrococcal nuclease (MNase)-hypersensitive regions of the chromatin. MNase-sensitive sites also undergo meiosis-specific alterations in chromatin structure prior to the appearance of DSBs. DSB formation requires the products of numerous genes. Herein we have examined the effects of mutations in four such genes, MRE11, RAD50, XRS2, and MRE2, on MNase sensitivity at DSB sites in premeiotic and meiotic cells. Disruption mutations in each of four genes confer greater than wild-type levels of MNase sensitivity in premeiotic cells. In meiotic prophase, all of these mutations affect MNase sensitivity at DSB sites and fall into two distinct phenotypic classes. The type 1 mutations (mre2 and mre11) confer a reduction in MNase sensitivity relative to the wildtype level. The type 2 mutations (rad50 and xrs2) permit a meiotic increase in the MNase sensitivity to reach a final level higher than that observed in wild-type cells. An mre11 disruption mutation (type 1) is epistatic to a rad50 null mutation (type 2) with respect to its meiotic effects on MNase sensitivity, suggesting that the events observed in the type 2 mutants during meiosis are dependent upon type 1 functions. One interpretation of these results is that Mre11, Rad50, Xrs2, and possibly Mer2 (whose splicing is Mre2-dependent) form a complex at recombination hot spots and establish a chromatin͞DNA configuration favorable for the induction of DSBs.Genetic recombination in eukaryotic organisms occurs at a frequency several orders of magnitude higher in meiotic cells than in somatic cells. In the yeast Saccharomyces cerevisiae, most meiotic recombination is initiated at defined sites by the formation of DNA double-stranded breaks (DSBs) that are subsequently repaired by recombination that occurs primarily between homologs (1-11).The induction of meiotic DSBs is affected by several factors (for reviews, see refs. 12-15). First, DSB formation is controlled by numerous genes including MER2, MRE2, MRE11, RAD50, SPO11, and XRS2 (3,[16][17][18][19][20][21][22][23][24][25][26][27][28]. In the corresponding null mutants, meiotic DSB formation is absent. Spo11 very likely functions as the catalytic subunit in the meiotic DNA cleavage reaction (29, 30). Mre2 (19) is required for the meiosis-specific splicing of the MER2 transcript (20). Rad50 and Xrs2 have been found to interact with Mre11 (21, 31), and it has been hypothesized that these three proteins required for mitotic DSBs repair form a recombination-initiating complex in meiosis that is essential for the pairing of homologs, DSB formation, and repair (32, 33). The Rad50 and Mre11 proteins are homologous to the Escherichia coli SbcC and SbcD nucleases, respectively (34, 35), which suggests that this complex may have nuclease activity. Furthermore, Rad50 is required for normal development of chromosome structure in meiosis (24) and also for telomere maintenance during mitosis (36).Second...
