DNA double-strand breaks (DSBs) are among the most deleterious type of DNA lesions threatening genome integrity. Homologous recombination (HR) and non-homologous end joining (NHEJ) are two major pathways to repair DSBs. HR requires a homologous template to direct DNA repair, and is generally recognized as a high-fidelity pathway. In contrast, NHEJ directly seals broken ends, but the repair product is often accompanied by sequence alterations. The choice of repair pathways is strictly controlled by the cell cycle. The occurrence of HR is restricted to late S to G2 phases while NHEJ operates predominantly in G1 phase, although it can act throughout most of the cell cycle. Deregulation of repair pathway choice can result in genotoxic consequences associated with cancers. How the cell cycle regulates the choice of HR and NHEJ has been extensively studied in the past decade. In this review, we will focus on the current progresses on how HR is controlled by the cell cycle in both Saccharomyces cerevisiae and mammals. Particular attention will be given to how cyclin-dependent kinases modulate DSB end resection, DNA damage checkpoint signaling, repair and processing of recombination intermediates. In addition, we will discuss recent findings on how HR is repressed in G1 and M phases by the cell cycle.

Repair of DNA double-strand breaks (DSBs) requires eviction of the histones around DNA breaks to allow the loading of numerous repair and checkpoint proteins. However, the mechanism and regulation of this process remain poorly understood. Here, we show that histone H2B ubiquitination (uH2B) promotes histone eviction at DSBs independent of resection or ATP-dependent chromatin remodelers. Cells lacking uH2B or its E3 ubiquitin ligase Bre1 exhibit hyper-resection due to the loss of H3K79 methylation that recruits Rad9, a known negative regulator of resection. Unexpectedly, despite excessive single-strand DNA being produced, bre1Δ cells show defective RPA and Rad51 recruitment and impaired repair by homologous recombination and response to DNA damage. The HR defect in bre1Δ cells correlates with impaired histone loss at DSBs and can be largely rescued by depletion of CAF-1, a histone chaperone depositing histones H3-H4. Overexpression of Rad51 stimulates histone eviction and partially suppresses the recombination defects of bre1Δ mutant. Thus, we propose that Bre1 mediated-uH2B promotes DSB repair through facilitating histone eviction and subsequent loading of repair proteins.

The coordination of DNA replication and repair is critical for the maintenance of genome stability. It has been shown that the Mrc1-mediated S phase checkpoint inhibits DNA double-stranded break (DSB) repair through homologous recombination (HR). How the replication checkpoint inhibits HR remains only partially understood. Here we show that replication stress induces the suppression of both Sgs1/Dna2- and Exo1-mediated resection pathways in an Mrc1-dependent manner. As a result, the loading of the single-stranded DNA binding factor replication protein A (RPA) and Rad51 and DSB repair by HR were severely impaired under replication stress. Notably, the deletion of MRC1 partially restored the recruitment of resection enzymes, DSB end resection, and the loading of RPA and Rad51. The role of Mrc1 in inhibiting DSB end resection is independent of Csm3, Tof1, or Ctf4. Mechanistically, we reveal that replication stress induces global chromatin compaction in a manner partially dependent on Mrc1, and this chromatin compaction limits the access of chromatin remodeling factors and HR proteins, leading to the suppression of HR. Our study reveals a critical role of the Mrc1-dependent chromatin structure change in coordinating DNA replication and recombination under replication stress.