The RNA-dependent DNA polymerase of avian sarcoma virus B77. Binding of viral and nonviral ribonucleic acids to the α, β₂, and αβ forms of the enzyme

The extent of binding of various RNA species to the three forms of avian sarcoma virus B77 RNA-dependent DNA polymerase was determined using a sensitive nitrocellulose filter binding technique which was capable of detecting binding reactions with association constants as low as 3 x 10⁶ liters x mol^-1. All three enzyme forms, αβ, β₂, and α, bound to all single-stranded RNA species that were tested, including nonviral RNAs. 70 S viral RNA exhibited the highest association constant (about 10¹¹ liters x mole^-1), and a population of virus-derived tRNA molecules from which tRNA(Trp) had been removed, the lowest (about 3000 times lower). The affinity for other RNAs was roughly proportional to their size. The affinity of RNAs for the αβ enzyme form always exceeded that for the two others by a factor that depended on the particular RNA, never exceeded 6 and was sometimes as low as 1.2. The association constant of the αβ enzyme form with viral 70 S RNA was about 15-fold higher than that with viral 35 S RNA. 35 S RNA annealed to tRNA(Trp) had an association constant that was only 2.5 times higher than that of 35 S RNA alone. This finding suggests that the tertiary structure of 70 S RNA plays a significant role in its affinity for B77 DNA polymerase.