The firing of eukaryotic origins of DNA replication requires CDK and DDK kinase 
activities. DDK, in particular, is involved in setting the temporal program of 
origin activation, a conserved feature of eukaryotes. Rif1, originally 
identified as a telomeric protein, was recently implicated in specifying 
replication timing in yeast and mammals. We show that this function of Rif1 
depends on its interaction with PP1 phosphatases. Mutations of two PP1 docking 
motifs in Rif1 lead to early replication of telomeres in budding yeast and 
misregulation of origin firing in fission yeast. Several lines of evidence 
indicate that Rif1/PP1 counteract DDK activity on the replicative MCM helicase. 
Our data suggest that the PP1/Rif1 interaction is downregulated by the 
phosphorylation of Rif1, most likely by CDK/DDK. These findings elucidate the 
mechanism of action of Rif1 in the control of DNA replication and demonstrate a 
role of PP1 phosphatases in the regulation of origin firing.