Zenvirth D, Loidl J, Klein S, Arbel A, Shemesh R, Simchen G
Department of Genetics, The Hebrew University of Jerusalem, Israel.
Genes Cells. 1997 Aug;2(8):487-98. doi: 10.1046/j.1365-2443.1997.1370335.x.
When Saccharomyces cerevisiae cells that have begun meiosis are transferred to mitotic growth conditions ('return-to-growth', RTG), they can complete recombination at high meiotic frequencies, but undergo mitotic cell division and remain diploid. It was not known how meiotic recombination intermediates are repaired following RTG. Using molecular and cytological methods, we investigated whether the usual meiotic apparatus could repair meiotically induced DSBs during RTG, or whether other mechanisms are invoked when the developmental context changes.
Upon RTG, the rapid disappearance of meiotic features--double-strand breaks in DNA (DSBs), synaptonemal complex (SC), and SC related structures-was striking. In wild-type diploids, the repair of meiotic DSBs during RTG was quick and efficient, resulting in homologous recombination. Kinetic analysis of double-strand breakage and recombination indicated that meiotic DSB formation precedes the commitment to meiotic levels of recombination. DSBs were repaired in RTG in dmc1, but not rad51 mutants, hence repair did not occur by the usual meiotic mechanism which requires the Dmc1 gene product. In haploids, DSBs were also repaired quickly and efficiently upon RTG, showing that DSB repair did not require the presence of a homologous chromosome. In all strains examined, SC and related structures were not required for DSB repair or recombination following RTG.
At least two pathways of DSB repair, which differ from the primary meiotic pathway(s), can occur during RTG: One involving interhomologue recombination, and another involving sister-chromatid exchange. DSB formation precedes commitment to recombination. SC elements appear to prevent sister chromatid exchange in meiosis.
当已开始减数分裂的酿酒酵母细胞转移至有丝分裂生长条件下（“恢复生长”，RTG）时，它们能够以高减数分裂频率完成重组，但会进行有丝分裂细胞分裂并保持二倍体状态。此前尚不清楚RTG后减数分裂重组中间体是如何修复的。我们使用分子和细胞学方法，研究了常规减数分裂机制是否能在RTG期间修复减数分裂诱导的双链断裂（DSB），或者当发育环境改变时是否会调用其他机制。
在RTG时，减数分裂特征——DNA双链断裂（DSB）、联会复合体（SC）及与SC相关的结构——迅速消失，这一点很显著。在野生型二倍体中，RTG期间减数分裂DSB的修复迅速且高效，导致同源重组。双链断裂和重组的动力学分析表明，减数分裂DSB的形成先于减数分裂水平重组的启动。在RTG过程中，dmc1突变体中的DSB能被修复，但rad51突变体中的不能，因此修复并非通过需要Dmc1基因产物的常规减数分裂机制发生。在单倍体中，RTG时DSB也能迅速且高效地被修复，表明DSB修复不需要同源染色体的存在。在所有检测的菌株中，RTG后DSB修复或重组不需要SC及相关结构。
在RTG期间，至少可发生两条不同于主要减数分裂途径的DSB修复途径：一条涉及同源染色体间重组，另一条涉及姐妹染色单体交换。DSB形成先于重组启动。SC元件似乎在减数分裂中阻止姐妹染色单体交换。
