Requirement of the avian retrovirus pp32 DNA binding protein domain for replication

The NH2-terminal amino acid sequence of the pp32 DNA binding protein has been determined, thus establishing its precise coding region in the polymerase gene of Rous sarcoma virus. Specific mutations were constructed in molecularly cloned Prague A DNA near the NH2- and COOH-termini of pp32 and the effects were assayed by transfection on chick embryo fibroblasts. Out-of-frame deletions at both sites and an in-frame deletion near the NH2 terminus rendered the DNA noninfectious and transformation negative. Single point mutations near the NH2 terminus reduced the transfection efficiency and the rate of virus replication. Biochemical studies indicated that the RNA-directed DNA polymerase and RNase H activities of the mutant viruses were not affected but the processing of the viral beta polypeptide was altered.