TELOMERASE REVERSE-TRANSCRIPTASE GENES IDENTIFIED IN TETRAHYMENA-THERMOPHILA AND OXYTRICHA-TRIFALLAX

Telomerase reverse transcriptase (TERT) has been identified as the cat alytic subunit of the chromosome end-replicating enzyme in Euplotes, y easts, and mammals. However, it was not reported among the protein com ponents of purified tetrahymena telomerase, the first telomerase ident ified and the most thoroughly studied. It therefore seemed possible th at Tetrahymena used an alternative telomerase that lacked a TERT prote in. We now report the cloning and sequencing of a Tetrahymena thermoph ila gene whose encoded protein has the properties expected for a TERT, including large size (133 kDa), basicity (calculated pi = 10.0), and reverse transcriptase sequence motifs with telomerase-specific feature s. The expression of mRNA from the Tetrahymena TERT gene increases dra matically at 2-5 h after conjugation, preceding de novo addition of te lomeres to macronuclear DNA molecules. We also report the cloning and sequencing of the ortholog from Oxytricha trifallax. The Oxytricha mac ronuclear TERT gene has no introns, whereas that of Tetrahymena has 18 introns, Sequence comparisons reveal a new amino acid sequence motif (CP), conserved among the ciliated protozoan TERTs, and allow refineme nt of previously identified motifs, A phylogenetic tree of the known T ERTs follows the phylogeny of the organisms in which they are found, c onsistent with an ancient origin rather than recent transposition. The conservation of TERTs among eukaryotes supports the model that telome rase has a conserved core (TERT plus the RNA subunit), with other subu nits of the holoenzyme being more variable among species.