Figure 5.

A model for telomere turnover in wild-type and mutant strains. Telomere length reflects a balance between elongation by telomerase (open right-pointing arrows) and an unknown activity (or activities) acting to shorten the telomere (solid left-pointing arrows). The sizes of arrows reflect the extent of activity. (A) Wild-type cells. Rap1p binds telomeric repeats and nucleates a specialized telomeric DNA–protein complex. A balance between telomerase action and limited telomere shortening activity (or activities) sets mean telomere length and the distribution of lengths within a narrow range. (B) rap1-ΔC cells. In the terminal zone of the telomere, accessibility to both telomerase and shortening activities (telomere turnover) is enhanced compared with wild-type RAP1 cells. (C) Terminally located Kpn mutant telomeric repeats (specified by ter1-Kpn telomerase), in the presence of rap1-ΔC, greatly enhance access to both telomerase and telomere degradation activities. Telomeres elongate and are degraded uncontrollably (deregulated telomeres). (D) Capping of previously deregulated telomeres by a functionally wild-type telomeric repeat restores normal cap function at the end of the telomere. This function is sufficient to reduce telomerase accessibility and cause both telomere lengthening and telomere shortening to become controlled, even though mean overall telomere length is markedly longer than wild type.