In Saccharomyces cerevisiae, the heteromeric kinase complex Cdc7p-Dbf4p pla
ys a pivotal role at replication origins in triggering the initiation of DN
A replication during the S phase. We have assayed the kinase activity of en
dogenous levels of Cdc7p kinase by using a likely physiological target, Mcm
2p, as a substrate. Using this assay, we have confirmed that Cdc7p kinase a
ctivity fluctuates during the cell cycle; it is low in the G(1) phase, rise
s as cells enter the S phase, and remains high until cells complete mitosis
. These changes in kinase activity cannot be accounted for by changes in th
e levels of the catalytic subunit Cdc7p, as these levels are constant durin
g the cell cycle. However, the fluctuations in kinase activity do correlate
with levels of the regulatory subunit Dbf4p. The regulation of Dbf4p level
s can be attributed in part to increased degradation of the protein in G(1)
cells. This G(1)-phase instability is cdc16 dependent, suggesting a role o
f the anaphase-promoting complex in the turnover of Dbf4p. Overexpression o
f Dbf4p in the G(1) phase can partially overcome this elevated turnover and
lead to an increase in Cdc7p kinase activity. Thus, the regulation of Dbf4
p levels through the control of Dbf4p degradation has an important role in
the regulation of Cdc7p kinase activity during the cell cycle.