Processivity assay using a homopolymeric RNA template. Processivity of the recombinant HIV-1 subtype B and C RT enzymes was assessed using homopolymeric RNA template poly (rA) and oligo (dT)12-18 DNA primer. The DNA primer was labeled with 32P at the 5'-terminus and annealed to poly (rA) RNA template at an equimolar ratio. Processivities were analyzed by monitoring the size distribution of DNA products in fixed-time experiments in the presence of heparin trap. Parallel reactions were run in the absence of trap to ensure that similar amounts of enzyme activities were present in the reactions. The sizes of some fragments of the standard are indicated on the left side of the panel. All reaction products were resolved by denaturing 6% polyacrylamide gel electrophoresis and visualized by phosphorimaging. M: molecular size standards. C: control reaction in which the heparin trap was preincubated with substrates before the addition of RT.