Inbar O, Kupiec M
Department of Molecular Microbiology and Biotechnology, Tel Aviv University, Ramat Aviv, Israel.
Curr Genet. 2000 Jul;38(1):23-32. doi: 10.1007/s002940000124.
Homologous recombination is an important DNA repair mechanism in vegetative cells. During the repair of double-strand breaks, genetic information is transferred between the interacting DNA sequences, thus creating a gene-conversion event. Gene conversion of a functional member of a gene family, which uses an inactive member (such as a pseudogene) as a template, might have deleterious consequences. It is therefore important for the cell to prevent recombination between divergent sequences. We have studied the repair of a double-strand break by recombination in a haploid yeast strain carrying 99% identical alleles located on different chromosomes. The fate of the broken chromosome was followed in the whole cell population without imposing selective constraints. Our results show that all the cells were able to repair the broken chromosome by gene conversion. During the repair, the cells arrest in the cell cycle with a "dumbbell" configuration characteristic of G2/M-arrested cells. Surprisingly, although all the cells repaired the broken chromosome, 60% of them were unable to resume growth and to form colonies after the repair was completed. The low level of cell recovery was due to the 1% divergence between the alleles, but was not dependent on the function of the mismatch-repair system. Cell death, however, could be prevented by the presence of an alternative source of perfect homology located on a different chromosome.
同源重组是营养细胞中一种重要的DNA修复机制。在双链断裂修复过程中，遗传信息在相互作用的DNA序列之间传递，从而产生基因转换事件。以无活性成员（如假基因）为模板的基因家族功能成员的基因转换可能会产生有害后果。因此，细胞防止不同序列之间的重组非常重要。我们研究了在携带位于不同染色体上99%相同等位基因的单倍体酵母菌株中，通过重组修复双链断裂的情况。在没有施加选择限制的情况下，追踪了整个细胞群体中断裂染色体的命运。我们的结果表明，所有细胞都能够通过基因转换修复断裂的染色体。在修复过程中，细胞会以G2/M期阻滞细胞特有的“哑铃”形态停滞在细胞周期中。令人惊讶的是，尽管所有细胞都修复了断裂的染色体，但其中60%在修复完成后无法恢复生长并形成菌落。细胞恢复率低是由于等位基因之间1%的差异，但不依赖于错配修复系统的功能。然而，位于不同染色体上的完美同源性的替代来源的存在可以防止细胞死亡。
