Molecular mechanism of meiotic recombination

在酵母中，基因重组在减数分裂中发生的次数是有丝分裂的1000倍。减数分裂重组的增加是由突触复合体（SC）和重组结节的形成提供的。为了阐明这些结构复合体在减数分裂重组中的作用，我们试图分离参与这些复合体形成的突变体。作为候选突变体，我们分离出新的突变体mre2、mre3、mre11，这些突变体在有丝分裂重组中表现良好，但在减数分裂重组中表现不足。对这些突变体的性状分析结果如下：当编码一种携带RNA结合蛋白一致氨基酸序列的蛋白质的MRE2基因出现缺陷时，可以观察到未配对染色体的形成，即轴向元件，但不会观察到突触复合物。此外，该突变体减数分裂的第一次分裂比野生型早1小时开始，而减数分裂的第二次分裂在减数分裂开始后的正常时间开始。突变体在第一次分裂时，纺锤杆体分离的距离是野生型的2倍。通过测量一对分离的主轴杆体之间的管道长度，证实了这一观察结果。这些结果表明MRE2参与了同源染色体的配对，同时也参与了减数分裂第一次分裂的开始。MRE3基因通过合成起始位点不同的两种mRNA表达两种不同的蛋白。一个是在有丝分裂中合成的，覆盖了MRE3基因的整个ORF。而另一个则是在减数分裂中特异性合成的，从ORF的近端开始。前一种蛋白质的功能是有丝分裂生长所必需的，另一种蛋白质的功能是减数分裂重组所必需的。在该基因的完全缺失突变体中，未观察到SC前体轴向元件的形成。发现MRE11对RAD50基因具有上位性，不仅参与在重组热点处引入减数分裂特异性双链断裂（DSBs），同时还参与DSBs的加工。少

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