The Est3 protein is a subunit of yeast telomerase.

EST1, EST2, EST3 and TLC1 function in a single pathway for telomere replication in the yeast Saccharomyces cerevisiae [1] [2], as would be expected if these genes all encode components of the same complex. Est2p, the reverse transcriptase protein subunit, and TLC1, the templating RNA, are subunits of the catalytic core of yeast telomerase [3] [4] [5]. In contrast, mutations in EST1, EST3 or CDC13 eliminate telomere replication in vivo [1] [6] [7] [8] but are dispensable for in vitro telomerase catalytic activity [2] [9]. Est1p and Cdc13p, as components of telomerase and telomeric chromatin, respectively, cooperate to recruit telomerase to the end of the chromosome [7] [10]. However, Est3p has not yet been biochemically characterized and thus its specific role in telomere replication is unclear. We show here that Est3p is a stable component of the telomerase holoenzyme and furthermore, association of Est3p with the enzyme requires an intact catalytic core. As predicted for a telomerase subunit, fusion of Est3p to the high affinity Cdc13p telomeric DNA binding domain greatly increases access of telomerase to the telomere. Est1p is also tightly associated with telomerase; however, Est1p is capable of forming a stable TLC1-containing complex even in the absence of Est2p or Est3p. Yeast telomerase therefore contains a minimum of three Est proteins for which there is both in vivo and in vitro evidence for their role in telomere replication as subunits of the telomerase complex.