Panyutin I G, Hsieh P
Genetics and Biochemistry Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892.
J Mol Biol. 1993 Mar 20;230(2):413-24. doi: 10.1006/jmbi.1993.1159.
DNA branch migration, a process whereby two homologous DNA duplexes exchange strands, is an essential component of genetic recombination. Models for homologous recombination have invoked spontaneous branch migration as one mechanism for the generation of large regions of heteroduplex DNA. During recombination, two homologous parental duplexes that contain similar, but not identical, sequences are paired and undergo strand exchange. An important issue is whether spontaneous branch migration is capable of traversing sequence heterology such as mismatches, insertions and deletions. We use a model four-strand system to examine the effect of mispaired or unpaired bases on branch migration. The assay consists of annealing two short duplexes having defined sequence heterologies. Following annealing, a Holliday junction is formed that is free to branch migrate. Our results demonstrate that a single base mismatch, insertion or deletion is sufficient to pose a substantial barrier to spontaneous branch migration. In the presence of magnesium, branch migration through such sequence heterologies is almost completely blocked. Others have shown that non-mobile four-way junctions undergo a dramatic shift in conformation in the presence of magnesium. Our data suggest that a similar transition occurs for the mobile Holliday junction. We also discuss how proteins may facilitate branch migration through sequence heterologies in vivo.
DNA分支迁移是指两条同源DNA双链交换链的过程，是基因重组的一个重要组成部分。同源重组模型认为自发分支迁移是产生大片段异源双链DNA的一种机制。在重组过程中，两条含有相似但不相同序列的同源亲本双链配对并进行链交换。一个重要的问题是自发分支迁移是否能够跨越序列异质性，如错配、插入和缺失。我们使用一个模型四链系统来研究错配或未配对碱基对分支迁移的影响。该实验包括退火两条具有特定序列异质性的短双链。退火后，形成一个可以自由进行分支迁移的霍利迪连接体。我们的结果表明，单个碱基错配、插入或缺失足以对自发分支迁移构成实质性障碍。在镁存在的情况下，通过这种序列异质性的分支迁移几乎完全被阻断。其他人已经表明，在镁存在的情况下，不可移动的四向连接体会发生构象的剧烈变化。我们的数据表明，可移动的霍利迪连接体也会发生类似的转变。我们还讨论了蛋白质在体内如何促进通过序列异质性的分支迁移。
