Lovett S T, Feschenko V V
Department of Biology, Brandeis University, Waltham, MA 02254-9110, USA.
Proc Natl Acad Sci U S A. 1996 Jul 9;93(14):7120-4. doi: 10.1073/pnas.93.14.7120.
A functional methyl-directed mismatch repair pathway in Escherichia coli prevents the formation of deletions between 101-bp tandem repeats with 4% sequence divergence. Deletions between perfectly homologous repeats are unaffected. Deletion in both cases occurs independently of the homologous recombination gene, recA. Because the methyl-directed mismatch repair pathway detects and excises one strand of a mispaired duplex, an intermediate for RecA-independent deletion of tandem repeats must therefore be a heteroduplex formed between strands of each repeat. We find that MutH endonuclease, which in vivo incises specifically the newly replicated strand of DNA, and the Dam methylase, the source of this strand-discrimination, are required absolutely for the exclusion of "homeologous" (imperfectly homologous) tandem deletion. This supports the idea that the heteroduplex intermediate for deletion occurs during or shortly after DNA replication in the context of hemi-methylation. Our findings confirm a "replication slippage" model for deletion formation whereby the displacement and misalignment of the nascent strand relative to the repeated sequence in the template strand accomplishes the deletion.
大肠杆菌中一种功能性甲基导向错配修复途径可防止在具有4%序列差异的101碱基对串联重复序列之间形成缺失。完全同源重复序列之间的缺失不受影响。两种情况下的缺失均独立于同源重组基因recA发生。由于甲基导向错配修复途径可检测并切除错配双链体的一条链，因此串联重复序列RecA非依赖性缺失的中间体必定是每个重复序列链之间形成的异源双链体。我们发现，体内特异性切割新复制DNA链的MutH内切核酸酶以及这种链区分的来源Dam甲基化酶，对于排除“不完全同源”（不完全同源）串联缺失绝对必要。这支持了缺失的异源双链体中间体在半甲基化背景下的DNA复制期间或之后不久出现的观点。我们的发现证实了缺失形成的“复制滑动”模型，即新生链相对于模板链中重复序列的位移和错配完成了缺失。
