Johnson-Schlitz Dena, Engels William R
Genetics Department, University of Wisconsin, Madison, WI 53706, USA.
Proc Natl Acad Sci U S A. 2006 Nov 7;103(45):16840-5. doi: 10.1073/pnas.0607904103. Epub 2006 Oct 30.
Previous biochemical studies of the BLM gene product have shown its ability in conjunction with topoisomerase IIIalpha to resolve double Holliday structures through a process called "dissolution." This process could prevent crossing over during repair of double-strand breaks. We report an analysis of the Drosophila BLM gene, DmBlm, in the repair of double-strand breaks in the premeiotic germ line of Drosophila males. With a repair reporter construct, Rr3, and other genetic tools, we show that DmBlm mutants are defective for homologous repair but show a compensating increase in single-strand annealing. Increases of 40- to 50-fold in crossing over and flanking deletions also were seen. Perhaps most significantly, the template used for homologous repair in DmBlm mutants is itself subject to deletions and complex rearrangements. These template disruptions are indicative of failure to resolve double Holliday junctions. These findings, along with the demonstration that a weak allele of topoisomerase IIIalpha has some of the same defects as DmBlm, support the dissolution model. Finally, an analysis of DmBlm mutants in conjunction with mus81 or spnA (Rad51) reveals a second function of BLM distinct from the repair of induced double-strand breaks and possibly related to maintenance of replication forks.
先前对BLM基因产物的生化研究表明，它能够与拓扑异构酶IIIα协同作用，通过一种称为“溶解”的过程来解析双Holliday结构。这一过程可以防止双链断裂修复过程中的交叉互换。我们报道了对果蝇BLM基因DmBlm在果蝇雄性减数分裂前生殖系双链断裂修复中的分析。利用一个修复报告构建体Rr3和其他遗传工具，我们发现DmBlm突变体在同源修复方面存在缺陷，但在单链退火方面有补偿性增加。在交叉互换和侧翼缺失方面也观察到了40至50倍的增加。也许最显著的是，DmBlm突变体中用于同源修复的模板自身也会发生缺失和复杂的重排。这些模板破坏表明无法解析双Holliday连接。这些发现，连同拓扑异构酶IIIα的一个弱等位基因具有与DmBlm一些相同缺陷的证明，支持了溶解模型。最后，对DmBlm突变体与mus81或spnA（Rad51）结合的分析揭示了BLM的第二个功能，该功能不同于诱导双链断裂的修复，可能与复制叉的维持有关。
