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Amy J. MacQueen


Molecular Biology & Biochemistry


Crossover recombination during meiosis is accompanied by a dramatic chromosome reorganization. In Saccharomyces cerevisiae homologous chromosomes enter meiosis unaligned, with their centromeres engaged predominantly in non-homologous associations; the onset of meiotic recombination by the Spo11 transesterase leads to stable pairwise associations between homologous centromeres followed by the intimate alignment of homologous axes via synaptonemal complex (SC) assembly. However, the molecular relationship between recombination and global meiotic chromosome reorganization remains poorly understood. In budding yeast, one question is why SC assembly initiates earliest at centromere regions while the DNA double strand breaks (DSBs) that initiate recombination occur genome-wide.

We targeted the site-specific HO endonuclease to various positions on S. cerevisiae’s longest chromosome, in order to ask whether a meiotic DSB’s proximity to the centromere influences its capacity to promote homologous centromere pairing and SC assembly (Chapter 2). We show that repair of an HO-mediated DSB does not promote homologous centromere pairing nor any extent of SC assembly in spo11 meiotic nuclei, regardless of its proximity to the centromere. DSBs induced en masse by phleomycin exposure likewise do not promote homologous centromere pairing nor robust SC assembly. Interestingly, in contrast to Spo11, HO-initiated interhomolog recombination is not constrained to use the meiosis-specific Dmc1 recombinase. Our results strengthen the previously proposed idea that specialized properties of Spo11 DSB machinery activate mechanisms that reinforce homologous chromosome alignment, and that two unique outcomes of Spo11-associated DSBs are homologous centromere pairing and the establishment of Dmc1 as the primary strand exchange enzyme.

DNA recombination matures within SC. Zip1 protein, a component of SC, promotes interhomolog crossovers. Although the absence of Zip1 shows a reduction in MutSg-mediated crossovers, how Zip1 promotes recombination is not known. A larger deletion of 244-511 amino acids in Zip1, zip1-M1, was previously implicated in crossover function. I examined crossovers in various zip1 alleles, that encode a protein with small ~10—20 amino acids deletion in Zipl’s M1 region. Our analysis shows that zip1-M1 may play an important role in crossover interference. We found that a smaller deletion in M1 region, zip1-B, formed excess crossovers suggesting this region may limit crossovers.



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