Background: Cdc7 kinase of Saccharomyces cerevisiae, a nuclear phosphoprotein, regulates initiation of chromosomal DNA replication. Overexpression of kinase-negative Cdc7 point mutants (T281E, D182N and D163N) arrests the cell cycle of the wild-type Saccharomyces cerevisiae cells at the G1/S boundary. This is caused by titration of a regulatory protein, Dbf4, from the wild-type Cdc7, which leads to inactivation of its kinase activity.
Results: We report here that kinase-negative Cdc7 mutants, when overexpressed in cdc28-1N(ts) at a permissive temperature, not only inhibit DNA replication by inactivating the wild-type Cdc7 but may also disturb coordination between DNA replication and cell division. Suppression of growth inhibition under this condition requires co-expression of both Dbf4 and Cdc28, whereas Dbf4 alone can counteract the growth inhibition in the wild-type cells. In cdc28-1N(ts), co-expression of the wild-type Dbf4 rescues only the G1/S defect and results in accumulation of those cells with less than 1C DNA as well as 2C DNA. On the other hand, co-expression of Cdc28 alone leads to increase of those cells arrested at the G1/S boundary, as found typically in the wild-type. We also report that overexpression of T281A, a 'weak' allele of Cdc7, causes growth arrest in cdc28-1N(ts) cells, but not in the CDC28 wild-type cells. This suggests that T281A is inactive in cdc28-1N(ts) and is consistent with the idea that Cdc28 activates Cdc7 by phosphorylation.
Conclusion: We conclude that two essential serine-threonine kinases, Cdc28 and Cdc7, genetically interact for initiation of the S phase and possibly for G2/M progression and/or S phase checkpoint control.