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A search for variant endogenous cat viruses led to a novel isolate. Although the major envelope glycoprotein of this virus was similar in size to that of an RD-114-like virus that was coisolated, it was unrelated to RD-114 or feline leukemia virus by immunological and biological criteria. This degree of dissimilarity suggests a different evolutionary progenitor from that for the RD-114 and feline leukemia virus viral envelopes. The novel virus did, however, code for gag gene polypeptides which are closely related to RD-114 virus. Neither the novel isolate nor the RD-114-like coisolate induced foci in S+L- cat cells which restrict focus induction by RD-114 virus. This suggests that the two viruses share a common genomic target of restriction which resides outside of the env region.

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Gazdar murine sarcoma virus (Gz-MSV) and Moloney murine sarcoma virus (M-MSV) are closely related. The complete M-MSV-specific nucleic acid sequences constituted a major portion of Gz-MSV-specific sequences. The MSV-specific sequences in both Gz-MSV and M-MSV genomes shared homology with hamster leukemia virus nucleic acid sequences. Both rat cells (S+L+) and hamster (S+L-) cells expressed two viral proteins of 68,000 and 70,000 daltons. These proteins were immunologically related to p60 purified from m1 virions of M-MSV.

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It was previously demonstrated that the 60,000 dalton (p60) precursor-like polyprotein containing murine p30 was a constituent of the feline leukemia virus pseudotype of Moloney sarcoma virus [m1MSV(FeLV)]. It is now shown that p60 is detected in cells of five mammalian species transformed by m1MSV, indicating that p60 is specified by this genome. Moreover, little or no murine p30 is detected in the m1MSV-transformed cells, suggesting that the murine group p30 antigenic reactivity of S + L- cells is ude to p60. Pulse-chase studies in cells producing m1MSV(FeLV) show that p60 is the largest polypeptide detectable during the pulse, and that intracellular p60 is not cleaved into smaller (for example, p30) polypeptides during chase periods of up to 10 hr. The lack of cleavage of p60 is in contrast to the properties of p30 precursors detected in cells containing replicating avian or mammalian RNA tumor viruses. The inefficient cleavage of intracellular p60 and the kinetics of appearance of murine p30 in extracellular m1MSV(FeLV) suggest that p60 cleavage to p30 occurs in cells shortly before virus release. While only p60 was detected in the m1MSV-transformed cells, p60 and p70 were detected in m3MSV-transformed cells, and no immunoprecipitable polypeptides were detected in HT-1 MSV-transformed cells. The observed differences in the intracellular polypeptide expression by each of the strains of MSV suggests differences in genetic content.

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Cat cells transformed by Moloney murine sarcoma virus contain virus-specific sequences in their RNA and DNA. Cloned, spontaneous revertant cell lines derived from clones of these cells had no evidence of the sarcoma genome in the cell RNA or DNA as judged by RNA-complementary DNA or DNA-complementary DNA hybridizations. ?This is apparently the first report of loss of a transforming genome in a revertant cell line.

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A 60,000-dalton polypeptide (p60) has been identified in the feline leukemia virus (FeLV) pseudotype of Moloney sarcoma virus [MSV(FeLV)]. This polypeptide is present in the purified virus complex in concentrations greater than either the murine p30 or the feline p27. Purified p60 crossreacts immunologically with murine p30 group antiserum and contains several interspecies determinants, whereas the group specific determinant of FeLV p27 is not detected. Comparison of peptide fingerprints of p60 and murine p30 show many peptides in common. Limited digestion of p60 with either trypsin or chymotrypsin produced p30-35 and p20 peptides which retain the MuLV p30 group and interspecies antigenic activities. The p30 produced by both enzymes comigrates in polyacrylamide gels with the murine p30 of MSV(FeLV), thus suggesting that p60 may be an uncleaved precursor to p30.

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Each of six mammalian C-type viruses-including two feline leukemia viruses, three murine leukemia viruses, and the human "candidate" virus RD-114-can be distinguished from each other by hybridizing DNA synthesized by viral reverse transcriptase with viral RNA. Characterization of the DNA . RNA hybrids by hydroxylapatite chromatography revealed nucleotide sequence diversity among the viruses, detectable both by the amount of cross-hybridization and by the decreased thermal stability of heterologous hybrids.

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A clone of cells derived from a continuous line of cat cells (CCC) spontaneously produced an RNA C-type virus (CCC virus) which did not have the group-specific antigen of the standard strains of feline leukemia viruses but did have that of the RD-114 virus. Single-hit infection of a virus yielding CCC cell with only the feline leukemia virus pseudotype of murine sarcoma virus [MSV(FeLV)] resulted in the release of a pseudotype of MSV coated with the CCC virus envelope. Host range, transmission of virus, helper functions, interference properties, and specific neutralization showed that the CCC and the RD-114 isolates as well as their respective MSV pseudotypes are closely similar if not identical. Parental, virus-negative cells frozen before the existence of RD-114 were chemically induced to yield CCC-like virus de novo. Infection of susceptible human cells with the chemically induced virus resulted in interference with the CCC virus pseudotype of MSV but not with the FeLV pseudotype of MSV.

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Murine sarcoma virus transformed mouse 3T3 cells, which are negative for murine leukemia virus and which yield sarcoma virus after superinfection with murine leukenmia virus, spotaneously give rise to flat variants front which murine sarcoma virus can no longer be rescued. The revertants support leukemia viruis growth and show an enhanced sensitivity to murine sarcoma superinfection and, like normal cells, do not release RNA-dependent DNA polymerase activity. Because revertants could be obtained with high frequency from progeny of single transformed cells, each cell that containts the sarconma virus genome seems to have the capacity to suppress or eliminate an RNA tumor virus native to its species of origin.

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Virions from hamster cells transformed by a new strain of murine sarcoma virus (MSV) appear to represent a form of MSV deficient in one or more of the viral components necessary for infectivity. Both the noninfectious virions and the sarcoma virus-transformed cells, which have no detectable replicating leukemia helper virus of either murine or hamster origin, are deficient in viral-type ribonucleic acid (RNA)-dependent deoxyribonucleic acid (DNA) polymerase activity (i.e., reverse transcriptase activity stimulated by the synthetic RNA-DNA template polyribo-adenylic-oligodeoxythymidylic acid). In comparison, both murine leukemia virus (MuLV) and MuLV-infected cells have abundant viral-type reverse transcriptase activity. It appears, therefore, that the leukemia virus genome may be required for the production of infectious sarcoma virus by contributing information needed for the full expression of functional viral-type reverse transcriptase.

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The nature of murine sarcoma virus (MSV) "defectiveness" was investigated by employing an MSV-transformed mouse 3T3 cell line which releases noninfectious virus-like particles. Rescue kinetics of MSV, observed after murine leukemia virus (MuLV) superinfection of these "sarcoma-positive leukemia-negative (S + L -)" mouse 3T3 cells, consisted of a 9- to 12-hr eclipse period followed by simultaneous release of both MSV and MuLV with no evidence for release of infectious MSV prior to the production of progeny MuLV. Addition of thymidine to the growth medium of MuLV-superinfected S + L - cells at a concentration suppressing deoxyribonucleic acid synthesis inhibited the replication of MuLV and the rescue of MSV. MSV production closely paralleled MuLV replication under a variety of experimental conditions. These results suggest that replication of MuLV is required for the rescue of infectious MSV from S + L - cells and that one (or more) factor, produced late in the MuLV replicative cycle, is utilized by both viruses during virion assembly. During the course of these experiments, virus stocks were recovered which contained infectious MSV in apparent excess over MuLV. These stocks were used for generating new S + L - cell lines by simple end point dilution procedures.

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Small numbers of virus-like particles were observed by electron microscopy in each of two cloned lines of 3T3 cells transformed by murine sarcoma virus, even though these lines were free of detectable quantities of infectious leukemia and sarcoma virus. The morphology and occurrence of the particles were identical to those of the murine leukemia-sarcoma group. Moreover, the particles incorporated uridine and had a buoyant density of 1.16 g/ml in sucrose gradients. No evidence of sarcoma or leukemia virus infectivity was associated with the particles in cells of several susceptible species under various conditions, including both cosedimentation with leukemia virus and infection in the presence of inactivated Sendai virus. The particles may represent a form of murine sarcoma virus deficient in one or more of the viral components necessary for infectivity.

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