Sugawara N, Ivanov E L, Fishman-Lobell J, Ray B L, Wu X, Haber J E
Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts 02254-9110.
Nature. 1995 Jan 5;373(6509):84-6. doi: 10.1038/373084a0.
In Saccharomyces cerevisiae, HO endonuclease-induced mating-type (MAT) switching is a specialized mitotic recombination event in which MAT sequences are replaced by those copied from a distant, unexpressed donor (HML or HMR). The donors have a chromatin structure inaccessible for both transcription and HO cleavage. Here we use physical monitoring of DNA to show that MAT switching is completely blocked at an early step in recombination in strains deleted for the DNA repair genes RAD51, RAD52, RAD54, RAD55 or RAD57. We find, however, that only RAD52 is required when the donor sequence is simultaneously not silenced and located on a plasmid. RAD51, RAD54, RAD55 and RAD57 are still required when the same transcribed donor is on the chromosome. We conclude that recombination in vivo occurs between DNA molecules in chromatin, whose structure significantly influences the outcome. RAD51, RAD54, RAD55 and RAD57 are all required to facilitate strand invasion into otherwise inaccessible donor sequences.
在酿酒酵母中，HO核酸内切酶诱导的交配型（MAT）转换是一种特殊的有丝分裂重组事件，其中MAT序列被从远处未表达的供体（HML或HMR）复制而来的序列所取代。供体具有一种对转录和HO切割均不可接近的染色质结构。在这里，我们利用对DNA的物理监测表明，在缺失DNA修复基因RAD51、RAD52、RAD54、RAD55或RAD57的菌株中，MAT转换在重组的早期步骤就被完全阻断。然而，我们发现，当供体序列同时不被沉默且位于质粒上时，仅需要RAD52。当相同的转录供体位于染色体上时，RAD51、RAD54、RAD55和RAD57仍然是必需的。我们得出结论，体内重组发生在染色质中的DNA分子之间，其结构显著影响结果。RAD51、RAD54、RAD55和RAD57对于促进链侵入原本不可接近的供体序列都是必需的。
