Lovett S T, Drapkin P T, Sutera V A, Gluckman-Peskind T J
Department of Biology, Brandeis University, Waltham, Massachusetts 02254-9110.
Genetics. 1993 Nov;135(3):631-42. doi: 10.1093/genetics/135.3.631.
In the genomes of many organisms, deletions arise between tandemly repeated DNA sequences of lengths ranging from several kilobases to only a few nucleotides. Using a plasmid-based assay for deletion of a 787-bp tandem repeat, we have found that a recA-independent mechanism contributes substantially to the deletion process of even this large region of homology. No Escherichia coli recombination gene tested, including recA, had greater than a fivefold effect on deletion rates. The recA-independence of deletion formation is also observed with constructions present on the chromosome. RecA promotes synapsis and transfer of homologous DNA strands in vitro and is indispensable for intermolecular recombination events in vivo measured after conjugation. Because deletion formation in E. coli shows little or no dependence on recA, it has been assumed that homologous recombination contributes little to the deletion process. However, we have found recA-independent deletion products suggestive of reciprocal crossovers when branch migration in the cell is inhibited by a ruvA mutation. We propose a model for recA-independent crossovers between replicating sister strands, which can also explain deletion or amplification of repeated sequences. We suggest that this process may be initiated as post-replicational DNA repair; subsequent strand misalignment at repeated sequences leads to genetic rearrangements.
在许多生物体的基因组中，缺失发生在长度从几千碱基对到仅几个核苷酸的串联重复DNA序列之间。通过基于质粒的检测方法来检测787碱基对串联重复序列的缺失，我们发现一种不依赖RecA的机制对即使是如此大的同源区域的缺失过程也有很大贡献。所测试的任何大肠杆菌重组基因，包括RecA，对缺失率的影响都不超过五倍。在染色体上存在的构建体中也观察到缺失形成不依赖RecA。RecA在体外促进同源DNA链的联会和转移，并且对于在接合后体内测量的分子间重组事件是必不可少的。由于大肠杆菌中的缺失形成几乎不依赖于RecA，因此人们认为同源重组对缺失过程的贡献很小。然而，当细胞中的分支迁移被ruvA突变抑制时，我们发现了不依赖RecA的缺失产物，提示存在相互交叉。我们提出了一个在复制的姐妹链之间不依赖RecA的交叉模型，该模型也可以解释重复序列的缺失或扩增。我们认为这个过程可能作为复制后DNA修复而启动；随后在重复序列处的链错配导致基因重排。
