Meiotic recombination in budding yeast

点击翻译按钮获取中文摘要

Fundamental to sexual reproduction is the ability to make gametes, such as sperm and eggs in humans, which contain only one copy of each chromosome. Fertilization then results in the fusion of two haploid gametes to create a diploid organism. For organisms such as budding yeast and humans, this is a daunting task, as there are 16 and 23 pairs of chromosomes, respectively, that must be properly sorted into each gamete. Meiosis is the specialized cell division that divides the chromosome number in half by having one round of DNA replication followed by two rounds of chromosome segregation. Failure to properly segregate chromosomes during meiosis produces chromosomally imbalanced gametes, resulting in infertility and birth defects such as Trisomy 21 (Down Syndrome). A critical part of meiosis is the first meiotic division, where homologous chromosomes are segregated to opposite poles of the spindle. Crossovers are created by the reciprocal exchange of DNA between homologs. Crossovers, in combination with sister chromatid cohesion, physically connect homologs so they can align and properly segregate at the first meiotic division. Crossovers result from the repair of double strand breaks that are deliberately introduced into chromosomes to initiate recombination. Because unrepaired double strand breaks are lethal, meiotic recombination is a highly regulated process that ensures that every pair of homologs receives at least one crossover and that all double strand breaks are repaired before the first meiotic division. Studying meiosis directly in mammals is difficult as it is hard to access germ cells and the cells are diploid, making it challenging to find recessive mutations. The budding yeast, Saccharomyces cerevisiae, has been an excellent model system for studying meiosis because of the sophisticated genetic, biochemical, molecular and cell biological tools that are available. The focus of my research has been on identifying genes required for meiotic recombination and defining the molecular mechanisms by which these genes function and/or are regulated. In particular, my lab has developed novel approaches for studying how phosphorylation regulates recombination during meiosis, with an emphasis on the meiosis-specific kinase, Mek1 and the conserved cell cycle kinase, Cdc7-Dbf4. Although immense progress has been made in our understanding of meiotic recombination, there are still critical gaps that need to be filled. For example, there are many genes that contribute to the fidelity of meiotic double strand break repair which remain to be discovered. Over the next five years, my lab plans to study two genes we have identified that were previously unknown to play a role in meiosis: SEN1 and RRM3. Sen1 is a helicase that unwinds RNA/DNA hybrids called R-loops in mitotically dividing cells and is essential for life. Its mammalian ortholog, Senataxin, is required for meiosis. Rrm3 is a member of the conserved Pif1 DNA helicase family that is well known for its role in DNA replication. In addition, we have discovered a potentially novel role for two other DNA helicases, Sgs1 and Srs2, working together in the regulation of crossover formation. Meiosis in many organisms such as yeast and mammals requires two recombinases, Rad51, which is essential for mitotic recombination and the meiosis-specific Dmc1, which mediates the bulk of interhomolog recombination. An outstanding question is why two recombinases are necessary. Work from my lab and others has suggested that Dmc1 has evolved to better handle the mismatched basepairs that can arise by interhomolog strand invasion because homologs have highly similar, but not necessarily identical DNA sequences. My lab has developed an in vivo approach to test this interesting hypothesis. The work supported by this grant will make an important contribution to our understanding of meiosis, knowledge which may ultimately be applicable in humans for preventing/treating infertility and birth defects.