Telomerase is processive

Telomerase synthesizes tandem repeats of the sequence d(TTGGGG)_n onto input d(TTGGGG)_n primer oligonucleotides (C. W. Greider and E. H. Blackburn, Cell 43:405-413). An intrinsic RNA component of the enzyme provides the template for d(TTGGGG)_n repeat synthesis [C. W. Greider and E. H. Blackburn, Nature (London) 337:331-337, 1989; G.-L. Lu, J. D. Bradley, L. D. Attardi, and E. H. Blackburn, Nature (London) 344:126-132, 1990]. In a typical reaction, products greater than 2,000 nucleotides were synthesized in 60 min. Dilution and primer challenge experiments showed that these long products were synthesized processively. The apparent processivity was not due to a higher affinity of the enzyme for long d(TTGGGG) products over the shorter competitors. The degree of processivity was quantitated; telomerase synthesized approximately 520 nucleotides before half of the enzyme had dissociated. After dissociating, telomerase reinitiated d(TTGGGG)_n synthesis on new primer oligonucleotides. The products from a telomerase reaction have a characteristic 6-nucleotide banding pattern (C. W. Greider and E. H. Blackburn, Cell 51:887-898, 1987). A strong pause in the reaction occurs after the addition of the first G in the sequence d(TTGGGG). Both the processivity and the banding pattern analysis imply that in the elongation mechanism there must be a translocation step after the 9 nucleotides of internal template RNA have been copied to the extreme 5′ end.