Genome Biology Department, Institute for Integrative Biology of the Cell (I2BC), CEA, CNRS, Université Paris-Sud, Université Paris-Saclay, Gif-sur-Yvette, France
Department of Biology, University of Copenhagen, Copenhagen, Denmark.
Microbiol Mol Biol Rev. 2018 Jun 13;82(3). doi: 10.1128/MMBR.00013-18. Print 2018 Sep.
In all organisms, replication impairments are an important source of genome rearrangements, mainly because of the formation of double-stranded DNA (dsDNA) ends at inactivated replication forks. Three reactions for the formation of dsDNA ends at replication forks were originally described for and became seminal models for all organisms: the encounter of replication forks with preexisting single-stranded DNA (ssDNA) interruptions, replication fork reversal, and head-to-tail collisions of successive replication rounds. Here, we first review the experimental evidence that now allows us to know when, where, and how these three different reactions occur in . Next, we recall our recent studies showing that in wild-type , spontaneous replication fork breakage occurs in 18% of cells at each generation. We propose that it results from the replication of preexisting nicks or gaps, since it does not involve replication fork reversal or head-to-tail fork collisions. In the mutant, deficient for double-strand break (DSB) repair, fork breakage triggers DSBs in the chromosome terminus during cell division, a reaction that is heritable for several generations. Finally, we recapitulate several observations suggesting that restart from intact inactivated replication forks and restart from recombination intermediates require different sets of enzymatic activities. The finding that 18% of cells suffer replication fork breakage suggests that DNA remains intact at most inactivated forks. Similarly, only 18% of cells need the helicase loader for replication restart, which leads us to speculate that the replicative helicase remains on DNA at intact inactivated replication forks and is reactivated by the replication restart proteins.
在所有生物体中，复制损伤是基因组重排的一个重要来源，主要是因为失活的复制叉会形成双链 DNA（dsDNA）末端。最初描述了三种在复制叉处形成 dsDNA 末端的反应，这些反应成为所有生物体的重要模型：复制叉与预先存在的单链 DNA（ssDNA）中断相遇、复制叉反转以及连续复制轮的头尾碰撞。在这里，我们首先回顾了实验证据，这些证据使我们现在能够知道这三种不同的反应在 中何时、何地以及如何发生。接下来，我们回顾了我们最近的研究表明，在野生型 中，每个细胞在每个世代都会自发地发生 18%的复制叉断裂。我们提出，这是由于预先存在的切口或缺口的复制所致，因为它不涉及复制叉反转或头尾碰撞。在 突变体中，双链断裂（DSB）修复缺陷，复制叉断裂会在细胞分裂过程中引发染色体末端的 DSB，这种反应在几代中是可遗传的。最后，我们总结了几个观察结果，表明从完整失活的复制叉和重组中间体重新启动需要不同的酶活性集。18%的细胞发生复制叉断裂的发现表明，大多数失活的复制叉处的 DNA 保持完整。同样，只有 18%的细胞需要解旋酶加载器进行复制重新启动，这使我们推测复制性解旋酶仍留在完整失活的复制叉上，并被复制重新启动蛋白重新激活。
