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MATERIALS AND METHODS Murine leukemia virus (MuLV). Several strains of mouse leukemia virus were used in these studies. Rauscher virus (R-MuLV) was grown in monolayer cultures of chronically infected mouse (BALB/c) bone marrow, JLS-V9 cells (50). The virus-shedding cell line was obtained from Electro-Nucleonics, Bethesda, Md. AKR virus was obtained from NIH Swiss embryo cells infected with an isolate from the spleen of an AKR mouse. These cells were kindly provided by Janet Hartley (National Cancer Institute). The La Puente isolate (1504E) of the wild mouse leukemia virus (WMLV) was obtained from Earle Officer (University of Southern California, Los Angeles, Calif.) (31; M.

The major internal virion polypeptide from feline and RD-114 type C viruses has been subjected to amino terminal sequence analyses with the Beckman automated sequencer. These proteins, as well as their homologs in rat and mouse viruses, begin with the sequence prolylleucylarginyl (Pro-Leu-Arg). Virus RD-114 differs from conventional feline type C viruses that show about 80 percent relatedness based on calculation of the minimum number of base changes to give equivalent coding for the protein segments analyzed. In addition, insertion of a gap in the RD-114 sequence is necessary to maintain positional homology. The difference between RD-114 and feline leukemia virus appears as great as the difference between mouse type C viruses and either of these two viruses. Thus, even though current evidence suggests that RD-114 is of feline origin, the sequence differences between RD-114 and conventional feline virus group-specific proteins is well beyond that based on one or a few point mutations.

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