A S/M DNA replication checkpoint prevents nuclear and cytoplasmic events of cell division including centrosomal axis alignment and inhibits activation of cyclin-dependent kinase-like proteins in fucoid zygotes

S/M checkpoints prevent various aspects of cell division when DNA has not b een replicated. Such checkpoints are stringent in yeast and animal somatic cells but are usually partial or not present in animal embryos, Because lit tle is known about S/M checkpoints in plant cells and embryos, we have inve stigated the effect of aphidicolin, a specific inhibitor of DNA polymerases alpha and delta, on cell division and morphogenesis in Fucus and Pelvetia zygotes, Both DNA replication and cell division were inhibited by aphidicol in, indicating the presence, in fucoid zygotes, of a S/M checkpoint. This c heckpoint prevents chromatin condensation, spindle formation, centrosomal a lignment with the growth axis and cytokinesis but has no effect on germinat ion or rhizoid elongation. This S/M checkpoint also prevents tyrosine depho sphorylation of cyclin-dependent kinase-like proteins at the onset of mitos is, The kinase activity is restored in extracts upon incubation with cdc25A phosphatase, When added in S phase, olomoucine, a specific inhibitor of cy clin-dependent kinases, has similar effects as aphidicolin on cell division although alignment of the centrosomal axis still occurs. We propose a mode l involving the inactivation of CDK-like proteins to account for the S/M DN A replication checkpoint in fucoid zygotes and embryos.