Telomerase: What Are the Est Proteins Doing?

Saccharomyces cerevisiae has proven to be a useful model organism for the study of telomerase, a specialized cellular reverse transcriptase that helps maintain genomic stability by adding telomeric DNA repeats to the ends of chromosomes. Yeast telomerase is thought to be a holoenzyme containing Est2p and TLC1 RNA, the catalytic subunit and its intrinsic template, respectively, as well as the TLC1-RNA-associated factors Est1p and Est3p. Cdc13p, a sequence-specific telomere-DNA-binding protein, is also required for action in vivo. A current model for telomerase regulation is that telomere-associated Cdc13p binds Est1p, thereby recruiting telomerase. However, recent chromatin immunoprecipitation experiments suggest an alternate role for Est1p in activating Est2p-TLC1-RNA that is already bound to the telomere. Three models for Est1p activation are presented.