Abstract

The Anaphase Promoting Complex (APC) is required for anaphase progression and B-type cyclin proteolysis. The recent finding that inactivation of the APC allows 'over-replication' of DNA has led to the proposal that the APC might also be required for preventing reduplication of chromosomes during G₂ and M phases. In this report we re-investigate the phenotype of apc mutant cells and find that they do not re-replicate their DNA during the period taken for wild-type cells to traverse G₂ and M phases. apc mutants do, however, gradually increase their DNA content after long periods of cell cycle arrest. Such DNA synthesis occurs almost exclusively in the cytoplasm and neither occurs in cells lacking mitochondrial DNA nor depends on Cdc6, a protein which is essential for the initiation of chromosomal but not mitochondrial DNA replication. ARS1, a chromosomal replication origin, is not re-fired in cells deprived of APC function, confirming that the 'over-replicated' DNA in apc mutant cells is of mitochondrial origin. Furthermore, we find that APC function is required to promote but not to prevent re-replication in ndc10 mutant cells. We therefore propose that the APC is not involved in preventing re-duplication of chromosomes during G₂ and M phases.