What is it about?
We investigated how Escherichia coli controls chromosome replication when extra replication starts away from the normal origin. We focused on RecG, PriA, and three single-stranded DNA exonucleases—ExoI, ExoVII and SbcCD—that can trim exposed DNA ends. We found that cells lacking RecG stayed alive only if at least one of these 3′ exonucleases was present; without RecG and all three exonucleases, cells were inviable. Removing PriA helicase activity restored viability, supporting a model in which RecG and these exonucleases limit PriA-driven re-replication and the DNA branches that can provoke recombination. Tests with RNase HI, RecA, RuvABC, Rep and UvrD helped place this mechanism alongside related issues such as RNA:DNA hybrids, mismatch repair and conflicts between replication and transcription.
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Why is it important?
The work suggests that finishing bacterial chromosome replication is more active and carefully controlled than a simple meeting of replication forks. Exposed single-stranded DNA generated during fork encounters may become harmful if it is not removed or remodelled. Our results support the idea that RecG and 3′ single-stranded DNA exonucleases help protect E. coli from inappropriate PriA-mediated replication and the recombination that follows.
Perspectives
What stands out to us is how several genetic tests pointed to the same underlying problem: DNA structures that are normally contained can become toxic when RecG and specific exonucleases are missing. It was especially informative that ExoI, ExoVII or SbcCD alone could maintain viability in recG cells, while removal of PriA helicase activity also rescued the key lethal combination. The study also helped us separate this core RecG–PriA–exonuclease relationship from other stresses seen in exonuclease-deficient cells, including elevated SOS response, mismatch repair defects and replication-transcription conflicts.
Dr. Christian J Rudolph
Brunel University
Read the Original
This page is a summary of: RecG Protein and Single-Strand DNA Exonucleases Avoid Cell Lethality Associated With PriA Helicase Activity in Escherichia coli, Genetics, July 2010, Genetics Society of America,
DOI: 10.1534/genetics.110.120691.
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