Properties and Origin of the Subunits of Reverse Transcriptase Isolated from Avian RNA Tumor Viruses

One of the best indicators for the RNA tumor viruses group is the presence of reverse transcriptase activity in the virion (1,11). This polymerase activity can efficiently copy different RNA templates to give DNA products. Reverse transcriptases were purified from several different viruses, and their structure and activity with various templates primers were studied (6,7,12). All the isolated viral enzymes show high activity when poly(C):oligo(dG) was used as a template-primer. Subsequently, this assay became a crucial test in the search for new reverse transcriptases in various viruses or cell extracts (2). The enzymes most thoroughly studied were isolated from avian RNA tumor viruses (AMV or RSV), murine leukemia viruses (MuLV), and hamster leukemia virus (HaLV). Reverse transcriptases isolated from the different viruses have different molecular structure. Enzymes purified from AMV and RSV were shown to be composed of two polypeptides: a small subunit with molecular weights of 55,000 to 65,000 and a large subunit with molecular weights of 90,000 to 100,000 (7). The enzyme isolated from HaLV was also shown to be a two-subunit enzyme with subunit molecular weights of 53,000 and 68,000 (14). The MuLV reverse transcriptase is a single polypeptide with a molecular weight of 70,000 (10). Only reverse transcriptase isolated from avian RNA tumor viruses is capable of efficiently copying 70 S RNA isolated from the virion.