During the proposed period of time we will investigate the following aspects of the role of reverse transcriptase during the process of reverse transcription. 1) Structural Studies on the Reverse Transcriptase: We plan to isolate the precursor of reverse transcriptase and study the steps involved in its processing to enzymatically active and stable form. We plan to study the relationship of two subunits comprising the hamster leukemia virus reverse transcriptase. We also plan to test our hypothesis that all reverse transcriptases are two subunit-containing enzymes. 2) Separation of DNA Polymerase and RNase H Activities: We want to physically separate the DNA polymerase and RNase H activities and study their individual properties. We plan to study the nature of the DNA synthesized by DNA polymerase devoid of RNase H activity. 3) Studies on the Temperature-Sensitive Mutants: We plan to identify the lesion in temperature-sensitive mutants defective in an early function. We also propose to study those mutants with defects in early function(s) but do not contain reverse transcriptase more thermolabile than the wild-type parent. 4) Intracellular Reverse Transcriptase: We propose to determine if reverse transcriptase-like protein is synthesized in nonproducer mammalian cells transformed by avian sarcoma viruses. We want to raise antibodies to purified reverse transcriptases from avian and murine RNA tumor viruses. Availability of monospecific antisera will allow us to detect very small amounts of reverse transcriptase-like protein in infected cells.