Abstract
Saccharomyces cells with one unrepaired double-strand break (DSB) adapt after checkpoint-mediated G2/M arrest. Adaptation is accompanied by loss of Rad53p checkpoint kinase activity and Chk1p phosphorylation. Rad53p kinase remains elevated in
yku70Δ and
cdc5-ad cells that fail to adapt. Permanent G2/M arrest in cells with increased single-stranded DNA is suppressed by the
rfa1-t11 mutation, but this RPA mutation does not suppress permanent arrest in
cdc5-ad cells. Checkpoint kinase activation and inactivation can be followed in G2-arrested cells, but there is no kinase activation in G1-arrested cells. We conclude that activation of the checkpoint kinases in response to a single DNA break is cell cycle regulated and that adaptation is an active process by which these kinases are inactivated.