Biosynthesis of reverse transcripta leukemia virus by synthesis and cle read-through viral precursor polyi (RNA-dependent DNA nucleotidyltransferase/processing of viral

Reverse transcriptase (RT; RNA-dependent DNA nucleotidyltransferase) from Rauscher leukemia virus is synthesized in infected cells by way of a read-through poly- rotein of 200,000 molecular weight. This polyprotein (Pr200 gag-pol) was precipitated by antiserum to RT; in a pre- vious study all the monospecific antisera to gag proteins rec- ognized Pr200 -p gag-pol Pr200 gag-poI contains both p30 and RT peptide sequences. Intermediate RT-related precursors of 145,000 (Prl45 Pol), 135,000 (Pr135 Po)), and 125,000 (Pr125 po0) molecular weights were specifically recognized by precipitation from infected cell extracts by antiserum to RT. These proteins shared methionine-containing tryptic peptide sequences with a virion polypeptide of 80,000 molecular weight (p80 P0o) pre- cipitable by antiserum to RT. Purification of active RT enzyme from virions labeled with (3H)methionine showed that p80 Pol was the major component, based on analysis by gel electro- phoresis and tryptic peptide mapping experiments. A polypep- tide (Pr80 Pol), similar in size to mature viral p80 P?l, was also precipitated from infected cells by antiserum to RT. Its peptide map was nearly identical to that of virion p80 Pol Pulse-chase studies showed that Pr80P?l, Pr125 P?, and Prl35 Po were stable polypeptides, whereas Pr200 gag-pol and Prl45 PO? were unstable precursors. Pulse-chase studies with the protein syn- thesis inhibitor, cycloheximide, showed that the processing of Pr200 gag-pol occurred for a short time in the absence of protein synthesis. Type-C retrovirus genomes contain genes for their internal structural proteins, reverse transcriptase (RT; RNA-dependent DNA nucleotidyltransferase), and envelope proteins.t These coding regions have been termed gag (group antigens), pol, and env, respectively (2). RT is present in much lower amounts in virus particles and in infected cells (3-5) than either the gag or env proteins. In this report we show that RT from Rauscher leukemia virus (RLV) is made in infected cells by synthesis and processing of a 200,000-dalton read-through protein containing both gag and pol determinants.