A NOVEL SPECIFICITY FOR THE PRIMER-TEMPLATE PAIRING REQUIREMENT IN TETRAHYMENA TELOMERASE

Telomerase is a specialized reverse transcriptase with a built-in RNA template. Base pairing between the templating domain of telomerase RNA and a telomeric DNA primer is normally a characteristic of elongation of telomeric DNA. Here we demonstrate the mechanism by which Tetrahym ena telomerase bypasses a requirement for template-primer pairing in o rder to add telomeric DNA de novo to completely non-telomeric DNA prim ers. We show that this reaction initiates by copying the template resi due at the 3' boundary of the telomerase RNA template sequence. Unexpe ctedly, as the RNA template moves through the telomerase catalytic cen ter, the number of required potential Watson-Crick base pairs between RNA template and DNA primer increases from zero to five. We propose th at this unprecedented position specificity of a base pairing potential requirement in a polymerase underlies the chromosome healing mechanis m of telomerase, and reflects constraints inherent in an internal temp late.