SUMMARY 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 Escherichia coli 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 E. coli. Next, we recall our recent studies showing that in wild-type E. coli, 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 recB 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.

总结在所有生物体中，复制障碍是基因组重排的重要来源，主要是因为双链 DNA (dsDNA) 末端在失活的复制叉处形成。在复制叉处形成 dsDNA 末端的三个反应最初是针对大肠杆菌描述的，并成为所有生物体的开创性模型：复制叉与预先存在的单链 DNA (ssDNA) 中断的相遇、复制叉逆转和头对-连续复制轮的尾部碰撞。在这里，我们首先回顾了现在让我们知道这三种不同反应何时、何地以及如何在大肠杆菌中发生的实验证据。接下来，我们回顾一下我们最近的研究表明，在野生型大肠杆菌中, 自发复制叉断裂发生在每一代的 18% 的细胞中。我们建议它是由先前存在的缺口或间隙的复制产生的，因为它不涉及复制叉反转或头对尾叉碰撞。在recB突变体，缺乏双链断裂（DSB）修复，叉断裂在细胞分裂期间触发染色体末端的DSB，这种反应可以遗传几代。最后，我们概括了几个观察结果，表明从完整的灭活复制叉重新启动和从重组中间体重新启动需要不同的酶活性集。18% 的细胞出现复制叉断裂这一发现表明，DNA 在大多数失活的复制叉处保持完整。同样，只有 18% 的细胞需要解旋酶加载器来重新启动复制，这使我们推测复制解旋酶在完整的失活复制叉处保留在 DNA 上，并被复制重新启动蛋白重新激活。