TETRAHYMENA TELOMERASE CATALYZES NUCLEOLYTIC CLEAVAGE AND NONPROCESSIVE ELONGATION

Telomerase is a ribonucleoprotein enzyme that adds telomeric repeats t o chromosomes, maintaining telomere length and stabilizing chromosome ends. In vitro, telomerase from the ciliate Tetrahymena elongates sing le-stranded, guanosine-rich DNA primers by adding repeats of the Tetra hymena telomeric sequence, dT2G4. We have identified two activities of Tetrahymena telomerase in addition to the previously described proces sive elongation reaction: a 3'-5' nucleolytic cleavage of primer or pr oduct DNA and a nonprocessive mode of elongation. The nucleolytic clea vage activity removed residues not conforming to the telomeric repeat sequence from a primer 3' end, eliminating mismatch between DNA primer and RNA template sequences. Template-matched residues were also cleav ed from primer or product DNA. Specific primer lengths, sequences, and concentrations stimulated cleavage and processive or nonprocessive el ongation differentially. These newly identified activities suggest tha t telomerase may catalyze a range of telomere synthesis and repair fun ctions and suggest mechanistic similarities between telomerase and RNA polymerase enzymes. On the basis of our results, we propose a model f or telomerase primer binding, cleavage, and elongation.