Farah Joseph A, Cromie Gareth, Steiner Walter W, Smith Gerald R
Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, Washington 98109-1024, USA.
Genetics. 2005 Mar;169(3):1261-74. doi: 10.1534/genetics.104.037515. Epub 2005 Jan 16.
DNA palindromes are rare in humans but are associated with meiosis-specific translocations. The conserved Mre11/Rad50/Nbs1 (MRN) complex is likely directly involved in processing palindromes through the homologous recombination pathway of DNA repair. Using the fission yeast Schizosaccharomyces pombe as a model system, we show that a 160-bp palindrome (M-pal) is a meiotic recombination hotspot and is preferentially eliminated by gene conversion. Importantly, this hotspot depends on the MRN complex for full activity and reveals a new pathway for generating meiotic DNA double-strand breaks (DSBs), separately from the Rec12 (ortholog of Spo11) pathway. We show that MRN-dependent DSBs are formed at or near the M-pal in vivo, and in contrast to the Rec12-dependent breaks, they appear early, during premeiotic replication. Analysis of mrn mutants indicates that the early DSBs are generated by the MRN nuclease activity, demonstrating the previously hypothesized MRN-dependent breakage of hairpins during replication. Our studies provide a genetic and physical basis for frequent translocations between palindromes in human meiosis and identify a conserved meiotic process that constantly selects against palindromes in eukaryotic genomes.
DNA回文序列在人类中很少见，但与减数分裂特异性易位有关。保守的Mre11/Rad50/Nbs1（MRN）复合物可能直接参与通过DNA修复的同源重组途径处理回文序列。使用裂殖酵母粟酒裂殖酵母作为模型系统，我们表明一个160bp的回文序列（M-回文）是减数分裂重组热点，并优先通过基因转换被消除。重要的是，这个热点的充分活性依赖于MRN复合物，并揭示了一条与Rec12（Spo11的直系同源物）途径不同的产生减数分裂DNA双链断裂（DSB）的新途径。我们表明，依赖MRN的DSB在体内M-回文处或其附近形成，与依赖Rec12的断裂不同，它们在减数分裂前复制期间早期出现。对mrn突变体的分析表明，早期DSB是由MRN核酸酶活性产生的，证明了之前假设的复制期间MRN依赖的发夹断裂。我们的研究为人类减数分裂中回文序列之间频繁的易位提供了遗传和物理基础，并确定了一个在真核基因组中不断筛选回文序列的保守减数分裂过程。
