Telomere length regulation in Saccharomyces cerevisiae

Telomere elongation is cell cycle-regulated and requires the coordinated activity of proteins involved in the DNA damage response. The cyclin-dependent kinase Cdk1 plays important roles in both cell-cycle progression and DNA-damage repair. We used an assay that detects de novo telomere addition to examine the role of the cyclin-dependent kinase Cdk1 (Cdc28) in cell-cycle specific telomere elongation. Inhibition of an ATP analogue-sensitive allele of Cdk1 completely blocked the addition of telomere repeats. Mutations in Rif2 and DNA polymerase alpha that cause increased telomere elongation were unable to compensate for the loss of Cdk1 activity, suggesting Cdk1 activity is required for an early step in telomere addition. Mutations in DNA repair proteins that act with Cdk1 at double-strand breaks also prevented telomere elongation. Cdk1 activity was required for the generation of 3' single-strand overhangs at both native and de novo telomeres. We propose Cdk1 activity controls the timing of telomere elongation by regulating the single-strand overhang at chromosome ends.; In addition to the role of the cyclin-dependent kinase Cdk1 in telomere elongation, the P13-like kinases Tel1 and Mec1 are required for normal telomere length maintenance. To identify the relevant telomere specific targets of these kinases, we developed a phosphorylation assay that allows us to identify specific telomere proteins that are phosphorylated in vivo. We found that the telomere end-binding protein Cdc13 and the telomere length regulator Rif1 are phosphorylated in vivo. The phosphorylation of Rif1 is cell cycle-regulated and depended on the activity of the cyclin-dependent kinase Cdk1. The phosphorylation of both Cdc13 and Rif1 is independent of Mec1/Tel1 and the DNA damage proteins Rad53 and Chk1. The precise amino acid targets that are phosphorylated in Cdc13 and Rif1 were not yet identified.