Progress has been made in the following areas: Initiation of meiotic recombination. Meiotic recombination is initiated by DNA double-strand breaks (DSBs);both the location and timing of break formation is tightly controlled.
Our aim i s to determine the substrate requirements of proteins that form DSBs, and the factors that control their location, frequency, and timing. Current research is directed at determining the chromosome structural elements that determine where DSBs do and do not form. We developed a novel technique to isolate intermediates in double-strand break-repair, based on their partially single-stranded DNA content. We applied this method to a microarray-based whole genome analysis to recombination intermediate distributions, and have determined the true distribution of meiotic recombination initiation events across the genome. Our data show that meiotic DSBs are much more evenly distributed than had been previously believed, and also illuminate the mechanism of DSB formation and repair during meiosis. Mechanism of meiotic recombination. We developed techniques to isolate and characterize unstable intermediates in meiotic recombination, and used these techniques to demonstrate that the two classes of meiotic recombination (events associated with crossing-over <I>versus</i>events not accompanied by crossing-over) proceed by distinct molecular mechanisms. More recently, we have focussed on a third class of meiotic recombination events--recombination between sister chromatids. Inter-sister recombination, which is the predominant form of homologous recombination during the mitotic cell cycle, was thought to be greatly reduced during meiosis, since interhomolog recombination is required to pair homologous chromosomes and to ensure their disjunction at the first meiotic division. We have now show that inter-sister recombination occurs much more frequently that previously thought during meiosis, and these findings are prompting a revision of the nature of recombination partner choice control. Recent experiments identify the Sgs1 helicase (the budding yeast homolog of the helicase mutated in Bloom's syndrome) and the Mus81/Mms4 structure-specific endonuclease as playing a critical role in controlling meiotic recombination. In the absence of these two proteins, abnormal recombination intermediates accumulate and block chromosome segregation. We are currently examining the role that other enzymes play in regulating meiotic recombination. Recent experiments have shown that the Yen1 structure-specific endonuclease is partially redundant with Mus81/Mms4 in resolving recombination intermediates, in that a subclass of intermediates accumulate unresolved in double mutants lacking both activities. We have also shown that the Sgs1-associated type 1 topoisomerase, Top3/Rmi1, is also essential to resolve another subclass of meiotic recombination intermediates. Regulation of progression through meiosis. We previously had shown that upregulation of a class of meiotically-expressed genes (middle-meiosis genes) was a critical step in the resolution of recombination intermediates as crossovers. We have now identified Cdc5, the budding yeast polo-like kinase, as the sole member of the >200 middle-meiosis genes that are required for both crossover formation and for the disassembly of meiosis-specific chromosome structures in advance of the meiotic divisions. We have now shown that Cdc5 is also required to shut off meiotic double-strand break formation prior to meiotic nuclear divisions. Polo kinases required a priming phosphorylation on substrates, and our search for the kinase the catalyzes this priming event has led to the discovery that both CDK (Cdc28 kinase + B-type cyclins) and DDK (Cdc7 kinase + Dbf4) activity are continuously required to maintain the integrity of meiosis-specific chromosome structures during meiosis I prophase. We have also taken advantage of a property of budding yeast meiosis, namely its reversibility, to examine mechanisms of recombination intermediates during the mitotic cell cycle. Recombination intermediates are accumulated in meiotic cells, which are then returned to mitotic growth conditions. In contrast to meiosis, where recombination intermediates are resolved primarily as crossover recombinants, these same intermediates are rapidly resolved predominantly as noncrossover upon return to growth. Recent experiments have identified the Sgs1 helicase as being required for this rapid non-crossover resolution.
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