Identification of the messenger RNAs coding for the gag and env gene products of the murine mammary tumor virus

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Full-length (35S) genomic RNA from murine mammary tumor virus (MuMTV) was translated in vitro, using a reticulocyte lysate system, into proteins of 105,000, 75,000, 65,000, 35,000, and 27,000 daltons. These proteins were all immunoprecipitable with a monospecific antiserum to the major viral core protein, p27, but not with antiserum to the major viral envelope glycoprotein, gp47. Translation in vitro of RNA of about 24S size extracted from MuMTV yielded proteins similar in size and immunoreactivity to the products of the 35S RNA translation. Polyadenylylated RNA isolated from an MuMTV-producing cell line was fractionated according to size by velocity sedimentation and subsequently hybridized to MuMTV complementary DNA probes. These studies identified at least three size classes (35S, 24S, and 14-18S) of intracellular MuMTV-specific RNA. The 35S intracellular RNA was translated into MuMTV-specific proteins identical in size and immunoreactivity to the products of the virion-derived 35S RNA. On the other hand, translation of the intracellular 24S RNA fraction resulted in the synthesis of proteins, of which two (of about 70,000 daltons) could be immunoprecipitated with anti-gp47 serum, but not with anti-p27 serum. From these data we conclude that MuMTV core and envelope proteins are synthesized from two different mRNAs with approximate sizes of 35S and 24S, respectively. Our results also imply that the intracellular 24S mRNA is synthesized by a process more complex than simple cleavage of the 35S RNA.

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