Proteolysis and tyrosine phosphorylation of p34cdc2/cyclin B -: The role of MCM2 and initiation of DNA replication to allow tyrosine phosphorylation of p34cdc2

Previously, it has been shown that Aspergillus cells lacking the function of nimQ and the anaphase-promoting complex (APC) component bimE(APC1) enter mitosis without replicating DNA, Here nimQ is shown to encode an MCM2 homologue, Although mutation of nimQ(MCM2) inhibits initiation of DNA replication, a few cells do enter mitosis, Cells arrested at G(1)/S by lack of nimQ(MCM2) contain p34(cdc2)/cyclin B, but p34(cdc2) remains tyrosine dephosphorylated, even after DNA damage, However, arrest of DNA replication using hydroxyurea followed by inactivation of nimQ(MCM2) and bimE(APC1) does not abrogate the S phase arrest checkpoint over mitosis, nimQ(MCM2), likely via initiation of DNA replication, is therefore required to trigger tyrosine phosphorylation of p34(cdc2) during the G(1) to S transition, which may occur by inactivation of nimT(cdc25), Cells lacking both nimQ(MCM2) and bimE(APC1) are deficient in the S phase arrest checkpoint over mitosis because they lack both tyrosine phosphorylation of p34(cdc2) and the function of bimE(APC1), Initiation of DNA replication, which requires nimQ(MCM2), is apparently critical to switch mitotic regulation from the APC to include tyrosine phosphorylation of p34(cdc2), at G(1)/S. We also show that cells arrested at G(1)/S due to lack of nimQ(MCM2) continue to replicate spindle pole bodies in the absence of DNA replication and can undergo anaphase in the absence of APC function.