Ionizing irradiation effects on S-phase in checkpoint mutants of the yeast Saccharomyces cerevisiae

In mammalian cells, gamma-irradiation activates checkpoint controls to delay entry into, or passage through S-phase, while chronic exposure to methyl methanesulfonate or hydroxyurea causes a similar delay in yeast. In yeast, at least five genes are involved: RAD9, RAD17, RAD24, RAD53 and MEC1, a homologue of ATM. Here, using flow cytometry analysis and alkaline sucrose gradient centrifugation of labeled, newly made DNA, we demonstrate, in synchronized RAD wild-type Saccharomyces cerevisiae cells, that: (1) gamma-irradiation at START delays entry into S-phase, (2) irradiation shortly before or during early S-phase delays completion of S-phase and (3) the latter response is largely a consequence of replicon initiation inhibition. The delay produced by irradiation during early S-phase depends on the function of the checkpoint genes RAD9, RAD17, RAD24, RAD53, MEC1 and MEC3. However, at least four, RAD17, RAD53, MEC1, MEC3, are not needed to delay S-phase progression when cells are irradiated shortly before S-phase begins.