G1-phase and B-type cyclins exclude the DNA-replication factor Mcm4 from the nucleus

Cyclin-dependent kinases (CDKs) activate the firing of replication origins during the S phase of the cell cycle, They also block re-initiation of DNA replication within a single cell cycle, by preventing the assembly of prere plicative complexes at origins. We show here that, in budding yeast, CDKs e xclude the essential prereplicative-complex component Mcm4 from the nucleus . Although origin firing can be triggered by the B-type cyclins only, both G1-phase and B-type cyclins cause exit of Mcm4 from the nucleus. These resu lts suggest that G1 cyclins may diminish the cell's capacity to assemble pr ereplicative complexes before B-type cyclins trigger origin firing during S phase.