Cell-cycle coordination between DNA replication and recombination revealed by a vertebrate N-end rule degron-Rad51.

Coordination of homologous DNA recombination (HDR) with DNA replication maintains the fidelity of cell division. Exploiting Varshavsky's N-end rule to create a thermosensitive degron for conditional genetics in an avian cell line, we confirm that inactivation of the essential HDR enzyme Rad51 in a single cell cycle does not stop replicative DNA synthesis but, instead, causes G2 arrest. Rad51 complementation after the completion of replication overcomes this block, suggesting that HDR becomes necessary in G2. Indeed, DNA structures that bind activated replication protein A accumulate during the S phase, to be preferentially resolved during G2 by a Rad51-dependent mechanism. Breaks affecting a single chromatid predominate after the first cell cycle without Rad51, subsequently evolving into isochromatid lesions. We suggest a model for the vertebrate cell cycle in which HDR during the G2 phase is separated from DNA replication in S phase and chromosome segregation in M.