The essential S-phase kinase Cdc7-Dbf4 acts at eukaryotic origins of replication to trigger a cascade of protein associations that activate the Mcm2-7 replicative helicase. Also known as Dbf4-dependent kinase (DDK), this kinase preferentially targets chromatin-associated Mcm2-7 complexes that are assembled on the DNA during prereplicative complex (pre-RC) formation. Here we address the mechanisms that control the specificity of DDK action. We show that incorporation of Mcm2-7 into the pre-RC increased the level and changes the specificity of DDK phosphorylation of this complex. In the context of the pre-RC, DDK preferentially targets a conformationally distinct subpopulation of Mcm2-7 complexes that is tightly linked to the origin DNA. This targeting requires DDK to tightly associate with Mcm2-7 complexes in a Dbf4-dependent manner. Importantly, we find that DDK association with and phosphorylation of origin-linked Mcm2-7 complexes require prior phosphorylation of the pre-RC. Our findings provide insights into the mechanisms that ensure that DDK action is spatially and temporally restricted to the origin-bound Mcm2-7 complexes that will drive replication fork movement during S phase and suggest new mechanisms to regulate origin activity.
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