Intermediates of DNA double strand break repair in escherichia coli
Mawer, Julia Sofia Pamela
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A DNA double-strand break (DSB) is a severe form of DNA damage. In fastgrowing cells, DSBs are commonly repaired by homologous recombination (HR) and in E. coli they are exclusively repaired by this mechanism. Failure to accurately repair DSBs can lead to genomic instability. Characterising the DNA intermediates formed during DSB repair by HR is key to understanding this process. A system for inducing a site-specific DSB in the E. coli chromosome has previously been described (Eykelenboom et al., 2008). Here, this system has been used to determine the nature of the intermediates of the repair. It was shown that in a Rec+ background the repair process is rapid and efficient. By contrast, in a ruvAB mutant, which is defective for the Holliday junction (HJ) migration and cleavage complex, RuvABC, HJs are accumulated on both sides of the breakpoint. Replication forks also accumulate at defined positions from the DSB, indicating that unresolved HJs are a barrier to efficient replication that is associated with the repair. This suggests that the resolution of HJs needs to occur prior to the establishment of DNA synthesis. Despite the accumulation of HJs in a ruvAB mutant, cell survival occurs when DSBs are induced for short periods, suggesting that HJs can be resolved in a RuvAB-independent manner. In contrast, the RecG helicase is essential for survival. In a recG mutant, replication forks but not HJs are detected in the region of DSB repair. In a ruvAB recG mutant, intermediates in this region are lost. These observations are consistent with a role of RecG in the stabilisation and maturation of D-loops and not the resolution of Holliday junctions. Nevertheless, an additional role for RecG in later stages of repair cannot yet be excluded. This work provides a solid framework for the further study of DSB repair in E. coli.