The RecBC complex protects single-stranded DNA gaps during lesion bypass

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Abstract
Following encounter with an unrepaired DNA lesion, replication is halted and can restart downstream of the lesion leading to the formation of a single-stranded DNA (ssDNA) gap. To complete replication, this ssDNA gap is filled in by one of the two lesion tolerance pathways: the error-prone Translesion Synthesis (TLS) or the error-free Homology Directed Gap Repair (HDGR). In the present work, we evidence a new role for the RecBC complex distinct from its canonical function in homologous recombination at DNA double strand breaks. We show that upon lesion encounter RecBC (independently of its catalytic activity and of the RecD subunit) is required to protect the nascent DNA in order to promote efficient lesion bypass. In the absence of RecBC, our data indicate that the nuclease ExoI can access and degrade the nascent DNA, affecting both TLS and HDGR mechanisms. We show that the recruitment of RecBC becomes particularly important at strong blocking lesions, when post-replicatively ssDNA gaps persist and are covered by the single-stranded DNA binding protein (SSB). This protective role of RecBC is reminiscent of the role of BRCA2 in protecting the nascent DNA in human cells, highlighting once again the evolutionary conservation of DNA replication mechanisms across all living organisms. SIGNIFICANCE STATEMENT: In the presence of unrepaired lesions, stalled replication can restart ahead of the lesion creating a single stranded DNA gap. To preserve genome stability, this gap needs to be later filled in by one of the two lesion tolerance pathways, Translesion Synthesis or Homology Directed Gap Repair. In this study, we unveil a role for the RecBC complex in protecting the nascent DNA blocked at the lesion site from degradation by the ExoI nuclease. This protection is essential for an efficient repair of the single-stranded DNA gap. In the absence of RecBC, both lesion tolerance pathways are affected and genome stability is compromised.
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