Varying the number of telomere-bound proteins does not alter telomere length in tel1 Delta cells

Yeast telomere DNA consists of a continuous, approximate to 330-bp tract of the heterogeneous repeat TG(1-3) with irregularly spaced, high affinity si tes for the protein Rap1p. Yeast monitor, or count, the number of telomeric Rap1p C termini in a negative feedback mechanism to modulate the length of the terminal TG1-3 repeats, and synthetic telomeres that tether Rap1p mole cules adjacent to the TG(1-3) tract cause wild-type cells to maintain a sho rter TG(1-3) tract. To identify trans-acting proteins required to count Rap 1p molecules, these same synthetic telomeres were placed in two short telom ere mutants: yku70 Delta (which lack the yeast Ku70 protein) and te/1 Delta (which lack the yeast ortholog of ATM). Although both mutants maintain tel omeres with approximate to 100 bp of TG(1-3), only ykv70 Delta cells mainta ined shorter TG(1-3) repeats in response to internal Rap1p molecules. This distinct response to internal Rap1p molecules was not caused by a variation in Rap1p site density in the TG(1-3) repeats as sequencing of tel1 Delta a nd yku70 Delta telomeres showed that both strains have only five to six Rap 1p sites per 100-bp telomere. In addition, the tel/1 Delta short: telomere phenotype was epistatic to the unregulated telomere length caused by deleti on of the Rap1p C-terminal domain. Thus, the length of the TG(1-3) repeats in tel/1 Delta cells was independent of the number of the Rap1p C termini a t the telomere. These data indicate that tel/1 Delta cells use an alternati ve mechanism to regulate telomere length that is distinct from monitoring t he number of telomere binding proteins.