Effects of different parameters on telomerase-mediated primer cleavage. (A) Effect of dGTP concentration on telomerase-mediated cleavage activity. DNA primer extension assays were conducted using different concentrations of dGTP. In addition to 1.25 µM [α-32P]dGTP, unlabeled dGTP was added. Lanes 1 and 3 contain no unlabeled dGTP. Lanes 2 and 4 contain 12.5 µM of unlabeled dGTP. (B) Cleavage of a telomeric primer containing a chain terminator (revdT) at the 3′-end. Direct DNA primer extension assays were performed with the indicated oligonucleotides. The 3′-end radiolabeled 5′-biotinylated primers, (T2AG3)3 and (T2AG3)4, migrate, respectively, as 19 and 25mers at the positions indicated by the arrows. ddA and ddT: dideoxyATP and dideoxyTTP. dGTP: deoxyGTP. Numbers in (A) and (B) represent the lengths of the elongation or chain termination products.