MUTATIONS IN THE MRE11, RAD50, XRS2, AND MRE2 GENES ALTER CHROMATIN CONFIGURATION AT MEIOTIC DNA DOUBLE-STRANDED BREAK SITES IN PREMEIOTIC AND MEIOTIC CELLS

In the yeast Saccharomyces cerevisiae, meiotic recombination is initia ted by DNA double-stranded breaks (DSBs) occurring in micrococcal nucl ease (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 a t DSB sites in premeiotic and meiotic cells. Disruption mutations in e ach of four genes confer greater than wild-type levels of MNase sensit ivity in premeiotic cells. In meiotic prophase, all of these mutations affect MNase sensitivity at DSB sites and fall into two distinct phen otypic classes. The type 1 mutations (mre2 and mre11) confer a reducti on in MNase sensitivity relative to the wild-type level, The type 2 mu tations (rad50 and xrs2) permit a meiotic increase in the MNase sensit ivity to reach a final level higher than that observed in wild-type ce lls. An mre11 disruption mutation (type 1) is epistatic to a rad50 nul l mutation (type 2) with respect to its meiotic effects on MNase sensi tivity, suggesting that the events observed in the type 2 mutants duri ng meiosis are dependent upon type 1 functions. One interpretation of these results is that Mre11, Rad50, Xrs2, and possibly Mer2 (whose spl icing is Mre2-dependent) form a complex at recombination hot spots and establish a chromatin/DNA configuration favorable for the induction o f DSBs.