Mechanisms and Regulation of Meiotic Recombination
Meiosis is a specialized cell cycle that produces haploid gametes from diploid parental cells, playing a central role in transmitting genetic material to the next generation. At meiosis I, homologous chromosomes segregate to the opposite poles while sisterchromatids remain associated. This meiosis- specific segregation pattern inevitably requires a mechanism to accurately recognize similarity between chromosomes so that homologous chromosomes align before separation.
Homologous recombination is a mechanism that detects similarity at the nucleotide level between two DNA molecules. Rad51 and Dmc1 are two orthologs of the bacterial RecA recombination protein. Unlike Rad51, which functions in both mitotic and meiotic cells, Dmc1 is produced only during meiosis.
By taking advantage of the powerful genetics available in budding yeast, together with microscopy and biochemistry, we study the mechanism of meiotic recombination and the role of recombination in homologous chromosome paring and segregation.
(1) The Rad51-Only Pathway Operates in Parallel to the Dmc1-Dependent Pathway during Meiosis
We demonstrated that there are two parallel pathways of meiotic recombination that differ in terms of their dependence on Rad51 and Dmc1 1. The pathway that predominates in meiotic cells (the Dmc1- dependent pathway) requires both recombinases, as well as a number of meiosis-specific proteins (as described below). The secondary pathway requires Rad51, but not Dmc1, and is similar to the pathway that operates to repair DNA double-strand breaks in vegetative cells.
(2) Identification of Proteins that Act with Dmc1 in Meiotic Recombination
We characterized a number of proteins that act in the Dmc1-dependent pathway of meiotic recombination. I identified a novel protein complex (the Hop2/Mnd1 complex) that discriminates homologous from non-homologous sequences during meiotic recombination and thus plays a major role in homologous chromosome recognition and alignment 1, 2. In addition, we found that two meiotic proteins, Mei5 and Sae3, work together with the meiosis-specific recombinase, Dmc1, 3.
(3) Discovery of Hed1, a Protein that inhibits Rad51 during Meiosis
We discovered a novel mechanism to down regulate the mitotic recombination machinery during meiosis when the meiotic recombination machinery is impaired 4, 5. A newly identified meiotic protein, Hed1, serves this function by directly binding and inhibiting Rad51, when Dmc1 is absent. Furthermore, since HED1 gene expression in vegetative cells inhibits mitotic recombination, Hed1 may serve as a tool to repress recombination in any cell type (such as higher eukaryotic cell lines and transgenic mice).
References
1. Tsubouchi, H. & Roeder, G. S. The importance of genetic recombination for fidelity of chromosome pairing in meiosis. Dev. Cell 5, 915-25 (2003).
2. Tsubouchi, H. & Roeder, G. S. The Mnd1 protein forms a complex with Hop2 to promote homologous chromosome pairing and meiotic double-strand break repair. Mol. Cell. Biol. 22, 3078- 3088 (2002).
3. Tsubouchi, H. & Roeder, G. S. The budding yeast Mei5 and Sae3 proteins act together with Dmc1 during meiotic recombination. Genetics 168, 1219-1230 (2004).
4. Tsubouchi, H. and G.S. Roeder. Budding yeast Hed1 down-regulates the mitotic recombination machinery when meiotic recombination is impaired. Genes Dev. 20, 1766-1775 (2006).
5. Busygina, V., Sehorn, M. G., Shi, I. Y., Tsubouchi, H., Roeder, G. S., and Sung, P. Hed1 regulates rad51-mediated recombination via a novel mechanism. Genes Dev. 22, 786-95 (2008).