Telomere length regulation: getting the measure of chromosome ends

Telomeres, the protein-DNA complexes that comprise the ends of linear eukaryotic chromosomes, serve to protect the chromosome ends and allow their complete replication. Telomeres also appear to play an essential role in chromosome segregation. In most organisms telomeric DNA consists of a series of short repeats that are variable in length, but regulated at a fixed average value in the germline. The possible involvement of telomere repeat shortening in aging and carcinogenesis has recently attracted attention to the more basic question of how telomere length is sensed and regulated by the cell. Telomere length in the budding yeast Saccharomyces cerevisiae has been known for over a decade now to be under complex genetic control, and this organism has provided a useful model system to address basic mechanistic questions. This review focuses on recent studies in yeast which indicate that the double-strand telomere-repeat binding protein Rap1 may play an important role in a negative-feedback mechanism that senses and controls the length of the telomere repeats. Although the same carboxy-terminal domain of Rap1p is involved in both telomere length regulation and telomeric silencing (telomere position effect), it appears that these two functions are mediated by separate sets of Rap1p-interacting proteins. Results from other systems suggest that negative regulation of telomere elongation by a double-stranded telomere-repeat binding protein may be a highly conserved strategy for telomere length control.