Nucleotide concentration dependence of the RTs from HIV-1, MuLV and R2. The assay involved an end-labeled 17-mer primer (P) annealed to either a 38-mer RNA template (a) or a DNA template (b). HIV-1, MuLV and R2 RT protein concentrations were adjusted to give 50% full extensions (F) in a 15 minute incubation at 37°C in the presence of 250 μM dNTPs. The same reactions were repeated with decreasing dNTP concentrations (lane 1, 250 μM; lane 2, 50 μM; lane 3, 5 μM; lane 4, 0.5 μM: lane 5, 0.05 μM; lane 6, 0.025 μM; lane 7, 0.0125 μM; andlane 8, 0.006 μM), and the prod ucts analyzed on 14% polyacrylamide-urea gels. R2 RT extensions longer than F were the result of this polymerase adding non-templated nucleotides (terminal transferase activity).24 Products labeled J in the R2 RT reactions were generated by the polymerase jumping from the 38-mer template to the excess 17-mer primer in the incubation and continuing the polymerization.24,25