Schildkraut Ezra, Miller Cheryl A, Nickoloff Jac A
Department of Molecular Genetics and Microbiology, University of New Mexico School of Medicine Albuquerque, NM 87131, USA.
Nucleic Acids Res. 2005 Mar 14;33(5):1574-80. doi: 10.1093/nar/gki295. Print 2005.
Homologous recombination (HR) repairs DNA double-strand breaks and maintains genome stability. HR between linked, direct repeats can occur by gene conversion without an associated crossover that maintains the gross repeat structure. Alternatively, direct repeat HR can occur by gene conversion with a crossover, or by single-strand annealing (SSA), both of which delete one repeat and the sequences between the repeats. Prior studies of different repeat structures in yeast and mammalian cells revealed disparate conversion:deletion ratios. Here, we show that a key factor controlling this ratio is the distance between the repeats, with conversion frequency increasing linearly with the distances from 850 to 3800 bp. Deletions are thought to arise primarily by SSA, which involves extensive end-processing to reveal complementary single-strands in each repeat. The results can be explained by a model in which strand-invasion leading to gene conversion competes more effectively with SSA as more extensive end-processing is required for SSA. We hypothesized that a transcription unit between repeats would inhibit end-processing and SSA, thereby increasing the fraction of conversions. However, conversion frequencies were identical for direct repeats separated by 3800 bp of transcriptionally silent or active DNA, indicating that end-processing and SSA are not affected by transcription.
同源重组（HR）修复DNA双链断裂并维持基因组稳定性。连锁的同向重复序列之间的HR可通过基因转换发生，而不伴有维持重复序列总体结构的交叉互换。另外，同向重复序列的HR也可通过伴有交叉互换的基因转换或单链退火（SSA）发生，这两种情况都会删除一个重复序列以及重复序列之间的序列。之前对酵母和哺乳动物细胞中不同重复序列结构的研究揭示了不同的转换与缺失比率。在此，我们表明控制这一比率的关键因素是重复序列之间的距离，转换频率随距离从850到3800 bp呈线性增加。缺失被认为主要由SSA产生，SSA涉及广泛的末端加工以在每个重复序列中暴露互补单链。这些结果可以用一个模型来解释，即随着SSA需要更广泛的末端加工，导致基因转换的链侵入与SSA的竞争更有效。我们假设重复序列之间的转录单元会抑制末端加工和SSA，从而增加转换的比例。然而，被3800 bp转录沉默或活跃DNA隔开的同向重复序列的转换频率是相同的，这表明末端加工和SSA不受转录的影响。
