Molecular mechanism of meiotic recombination

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Genetic recombination in yeast, S. cerevisiae, ocurs about 1000 times in meiosis than in mitosis. This increment in meiotic recombination is provided by the formation of the synaptonemal complex (SC) and recombination nodule. To clarify the roles of these structural complexes in meiotic recombination, we attempted to isolate the mutants involved in the formation of these complexes. As the candidates, we isolated new mutants, mre2, mre3, mre11, that area proficient in mitotic recombination but defective in meiotic recombination. Analysis of the properties of these mutants showed the following results. When the MRE2 gene, which encodes a protein carrying a consensus amino acid sequence for RNA binding proteins, is defective, the formation of the unpaired chromosomes, that is, axial elements, were observed but not the synaptonemal complex. Moreover, in this mutant the meiotic first division starts one hour earlier than in wild type, but the meiotic second division starts at a normal timin … More g after the onset of the meiosis. During the first division in the mutant, distance of the separated spindle pole body increased two times of that in wild type. This observation was confirmed by the measuring the lengths of the tublines between a pair of the divided spindle pole bodies. These results suggest that the MRE2 is involved in the pairing of the homologous chromosomes and also in the onset of the meiotic first division, simultaneously. The MRE3 gene expresses two different proteins by synthesizing two kinds of mRNA differing in the starting sites. One is synthesized in mitosis and covered the whole ORF of the MRE3 gene. While, the another is synthesized in meiosis specifically and starts form the proximal part within the ORF. The function of the former protein is required for the mitotic growth and that of the another for meiotic recombination. In the complete deletion mutant of this gene, the formation of the axial elements, precursors of the SC, was not observed. The MRE11 is found to be epistatic to the RAD50 gene and is involved in not only the introduction of the meiosis specific double strand breaks (DSBs) at the recombination hot spots but also the processing of the DSBs simultaneously. Less