REVERSE-TRANSCRIPTASE MOTIFS IN THE CATALYTIC SUBUNIT OF TELOMERASE

Telomerase is a ribonucleoprotein enzyme essential for the replication of chromosome termini in most eukaryotes. Telomerase RNA components h ave been identified from many organisms, but no protein component has been demonstrated to catalyze telomeric DNA extension. Telomerase was purified from Euplotes aediculatus, a ciliated protozoan, and one of i ts proteins was partially sequenced by nanoelectrospray tandem mass sp ectrometry. Cloning and sequence analysis of the corresponding gene re vealed that this 123-kilodalton protein (p123) contains reverse transc riptase motifs. A yeast (Saccharomyces cerevisiae) homolog was found a nd subsequently identified as EST2 (ever shorter telomeres), deletion of which had independently been shown to produce telomere defects. Int roduction of single amino acid substitutions within the reverse transc riptase motifs of Est2 protein led to telomere shortening and senescen ce in yeast, indicating that these motifs are important for catalysis of telomere elongation in vivo. In vitro telomeric DNA extension occur red with extracts from wild-type yeast but not from est2 mutants or mu tants deficient in telomerase RNA. Thus, the reverse transcriptase pro tein fold, previously known to be involved in retroviral replication a nd retrotransposition, is essential for normal chromosome telomere rep lication in diverse eukaryotes.