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.Several continuous lines of mouse embryo cells are susceptible to transformation and focus formation by murine sarcoma virus (MSV) (1, 2). Titration patterns of MSV in various isolates of mouse 3T3 cells indicate that focus formation in monolayer cultures under the usual assay conditions is dependent on dual infection by MSV and a murine leukemia virus (MuLV), which acts as a "helper" (1, 2). Thus, the behavior of MSV in mouse 3T3 cells, under certain culture conditions, is identical to that originally reported for MSV focus formation in mouse embryo cells (3). However, when culture methods are used that permit the multiplication of single transformed cells, while restricting reinfection of cells with progeny virus, both cell transformation and focus formation by MSV occur in the absence of MuLV "helper" virus and the properties of 3T3 cells infected with MSV alone can be examined (2,4).Growth of cells in semisolid agar suspension cultures is one such selective procedure that favors the multiplication of transformed cells and, at the same time, restricts virus spread (5). When 3T3 cells are infected with the Moloney isolate of MSV and plated as suspension cultures in semisolid agar, colonies of transformed cells can be seen after a suitable incubation period (2). The quantitative aspects of colony formation in this system indicate that cell transformation depends only on infection with MSV and occurs independently of infection with MuLV "helper" virus. Indeed, certain lines of MSV-transformed cells isolated from individual semisolid agar colonies contain a rescuable MSV genome in the absence of leukemia virus replication and have been designated "sarcoma-positive, leukemia-negative (S+L-)" (2). S+L-cells do not contain detectable quantities of focus-forming MSV unless superinfected with MuLV, after which both MSV and progeny MuLV are readily recovered (2). Transformation of mouse cells by MSV, therefore, does not require the replication of MuLV, and the "helper" activity of MuLV must involve either a quantitative or a qualitative effect on MSV replication.In addition to semisolid agar suspension cultures, monolayer cultures of 3T3 cells have also been us...

RNA from noninfectious virions produced by two established clonal -lines of sarcoma positive-leukemia negative (S+L-)-transformed 3T3 cells has been characterized. RNA from virions or nucleoids of S+L--(C243) cells consisted of three to four sizes: +44 S (6%), 28 S (17%), 18 S (38%), and <18 S (39%). 28S Genomes of RNA tumor viruses have been studied extensively. Nondissociated genomes (-60-70 S), which can be dissociated to smaller RNAs (c30-40 S), have been isolated from sarcoma-leukemia complexes as well as leukemia viruses of murine (1), avian (2, 3), feline (4), and other RNA tumor viruses (5). Genomes (-60-70 S) of transforming and nontransforming variants of Rous sarcoma virus (RSV) dissociate primarily into "a" and "b" subunits, respectively (6).The wild-type isolate of murine Moloney sarcoma virus (MSV) and avian RSV generally consisted of complexes containing sarcoma virus with an excess of leukemia virus (7,8), with the noticeable exceptions of the following avian RNA tumor viruses: B-RSVa(O), B-RSV3(O), SR-RSV, PR-RSV, and B-77 (6, 9).With the isolation and characterization of the sarcoma positive-leukemia negative (S+L-)-transformed 3T3 particleand antigen-producing cells (10, 11), as well as the sarcoma positive-helper negative (S+H -)-transformed hamster "producer" cells (12, 13), it became possible to characterize the genome from S-+fL-murine virions, as has been done for avian RNA tumor viruses (9). This research was initiated to characterize the RNA of S+L-virions.

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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 MSVspecific 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 ml virions of M-MSV. Several murine sarcoma viruses with distinct unique sequences have been isolated (5, 7, 9). Gazdar isolated a murine sarcoma virus (Gz-MSV) from a spontaneous tumor in a NZW/ NZB mouse (3). The biological properties of the virus were found to be very similar to Moloney murine sarcoma virus (M-MSV) (2-4). The present investigation was undertaken to compare the nucleotide sequences of Gz-MSV with those in M-MSV by nucleic acid hybridization and analysis of viral expression in cells. In the present communication, we show that Gz-MSV and M-MSV are closely related. A portion of the MSV-specific sequences in Gz-MSV was not found in M-MSV, although the complete MSVspecific sequences in M-MSV were shown to be present in Gz-MSV. In addition, both MSV-specific sequences of both Gz-MSV and M-MSV shared homology with hamster leukemia viruses (HaLV). Gz-MSV did not appear to recombine with rat endogenous virus sequences in spite of its propagation in rat cells. In addition, both the rat and hamster cells infected with Gz-MSV expressed two viral proteins of 68,000 and 70,000 daltons. These proteins were immunoprecipitated by anti-p60 serum prepared from M-MSV(FeLV) virus. MATERUILS AND METHODS Cell lines and viruses. Rat tumor-Gazdar (RTG-1:S+L+) cells infected with murine leukemia virus and Gazdar murine sarcoma virus (Gz-MSV/MuLV) and hamster tumor-Gazdar (HTG-2:S+L-) cells infected with defective Gz-MSV(HaLV) were used. F-2833 Graffi hamster cells, infected with chemically induced HaLV, were a gift from Paul Price (Microbiological Associates, Bethesda, Md.). The species of origin of cells were confirmed by karotype and isoenzyme analyses. Moloney MuLV (1869) was propagated in Scl cells. 3B11-IC mouse cells, infected with the