dNTP concentration dependence of single-cycle processivity of WT and mutant RTs. The DNA primer dPR was 5'-end labeled with [γ-32P]ATP and annealed to HIV PBS RNA. Extension was performed using a heparin trap and equivalent amounts of recombinant RTs at three different dNTP concentrations: 200 μM, 5 μM, and 2 μM. The sizes of some fragments of the 32P-labeled 10 bp DNA ladder (Invitrogen) in nucleotide bases are shown on the left. Positions of 32P-labeled dPR primer (32P-dPR) and full-length extension product (FL DNA) are indicated on the right.