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During S phase in Saccharomyces cerevisiae, chromosomal loci become mobile in response to DNA double-strand breaks both at the break site (local mobility) and throughout the nucleus (global mobility). Increased nuclear exploration is regulated by the recombination machinery and the DNA damage checkpoint and is likely an important aspect of homology search. While mobility in response to DNA damage has been studied extensively in S phase, the response in interphase has not, and the question of whether homologous recombination proceeds to completion in G1 phase remains controversial. Here, we find that global mobility is triggered in G1 phase. As in S phase, global mobility in G1 phase is controlled by the DNA damage checkpoint and the Rad51 recombinase. Interestingly, despite the restriction of Rad52 mediator foci to S phase, Rad51 foci form at high levels in G1 phase. Together, these observations indicate that the recombination and checkpoint machineries promote global mobility in G1 phase, supporting the notion that recombination can occur in interphase diploids.

Cellular responses to DNA damage can prevent mutations and death. In this study, we have used high-throughput screens and developed a comparative genomic approach, termed Functionome mapping, to discover conserved responses to UVC-damage. Functionome mapping uses Gene Ontology (GO) information to link proteins with similar biological functions from different organisms, and we have used it to compare 303, 311 and 288 UVC-toxicity modulating proteins from E. coli, S. pombe and S. cerevisiae, respectively. We have demonstrated that all three organisms use DNA repair, translation and aerobic respiration associated processes to modulate the toxicity of UVC, with these last two categories highlighting the importance of ribosomal proteins and electron transport machinery. Our study has demonstrated that comparative genomic approaches can be used to identify conserved responses to damage, and suggest roles for translational machinery and components of energy metabolism in optimizing the DNA damage response.

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Homologous recombination (HR), a principal cellular pathway for double-strand break (DSB) repair, is linked to changes in chromosome movement. Although increased chromosome mobility in response to a DSB has been observed in a variety of species, its precise role in HR remains controversial. Here, we find that end resection, the recruitment of recombination proteins, increased chromosome mobility, the pairing of homologs and gene conversion are temporally linked in response to a DSB. In mre11Δ mutant cells, which exhibit a delay in the initial processing of a DSB, chromosome mobility and all subsequent recombination events are also delayed. Overexpression of the Dna2 nuclease suppresses the mre11Δ delay in end resection and restores the original timing of chromosome mobility and all subsequent downstream HR events. Thus, changing the timing of chromosome mobility results in a corresponding change in essential downstream HR events, reinforcing its mechanistic role in the DNA repair process.

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