Recovery of avian sarcoma virus from tumors induced by transformation-defective mutants

Evidence for the common origin of viral and cellular sequences involved in sarcomagenic transformation

The src genes of six different strains of avian sarcoma virus (ASV) were compared with those of a series of newly isolated sarcoma viruses, termed "recovery avian sarcoma viruses" (rASV's). The rASV's were isolated recently from chicken and quail tumors induced by transformation-defective (td) deletion mutants of Schmidt-Ruppin Rous sarcoma virus. The RNase T1-resistant oligonucleotide maps were constructed for the RNA genomes of different strains of ASV and td mutants. The src-specific sequences, characterized by RNase T1-resistant oligonucleotides ranging from 9 to 19 nucleotides long, were defined as those mapping between approximately 600 and 2,800 nucleotides from the 3' polyadenylate end of individual sarcoma viral RNAs, and missing in the corresponding td viral RNAs. Our results revealed that 12 src-specific oligonucleotides were highly conserved among several strains of ASV, including the rASV's, whereas certain strains of ASV were found to contain one to three characteristic src-specific oligonucleotides. We previously presented evidence supporting the idea that most of the src-specific sequences present in rASV RNAs are derived from cellular genetic information. Our present data indicate that the src genes of rASV's are closely related to other known ASVs. We conclude that the src genes of different strains of ASV and the cellular sarc sequences are of common origin, although some divergence has occurred among different viral src genes and related cellular sequences.

Transforming activity of DNA of chemically transformed and normal cells

DNA fragments of chemically transformed and normal avian and murine cells induce transformation of NIH 3T3 mouse cells with low efficiencies. High molecular weight DNAs of cells transformed by DNA fragments induce transformation with high efficiencies in secondary transfection assays. It thus seems that endogenous transforming genes of uninfected cells can be activated and efficiently transmitted by transfection. These results are consistent with the hypothesis that normal cells contain genes that are capable of inducing transformation if expressed at abnormal levels.

Relationship of polypeptide products of the transforming gene of Rous sarcoma virus and the homologous gene of vertebrates

All vertebrate cells have been shown to contain a gene, sarc, that has some homology with the transforming gene of Rous sarcoma virus, src. We have compared the polypeptide products of the sarc gene, p60(sarc), of human, mouse, and chicken cells with the polymorphic polypeptide product of the src gene, p60(src), of several strains of Rous sarcoma virus by two-dimensional peptide mapping. p60(sarc) from chicken cells was clearly related to every viral p60(src). Eleven of its 13 methionine-containing tryptic peptides were present in some viral p60(src). Conversely, the other two peptides were not present in any p60(src) we have examined so far. The 11 peptides from p60(sarc) of chickens that were shared with viral p60(src), however, were not all present in any single viral p60(src). These 11 peptides most closely resemble those in the p60(src)s of B77 virus and the Prague strain of Rous sarcoma virus. These data are consistent with the hypothesis that cellular sarc is the progenitor of viral src. The p60(sarc)s of human, mouse, and chicken cells were so similar in tryptic peptide composition that they were more closely related to each other than were some viral p60(src)s. The two mammalian p60(sarc)s differed from avian p60(sarc) most notably in that they lacked a peptide that chicken p60(sarc) shares with all the viral p60(src)s. The similarity of these maps suggests that the sequence of the p60(sarc) polypeptide has diverged very little during evolution. This may imply that p60(sarc) is an essential cellular component.

Transforming gene product of Rous sarcoma virus phosphorylates tyrosine

The protein kinase activity associated with pp60src, the transforming protein of Rous sarcoma virus, was found to phosphorylate tyrosine when assayed in an immunoprecipitate. Despite the fact that a protein kinase with this activity has not been described before, several observations suggest that pp60src also phosphorylates tyrosine in vivo. First, chicken cells transformed by Rous sarcoma virus contain as much as 8-fold more phosphotyrosine than do uninfected cells. Second, phosphotyrosine is present in pp60src itself, at one of the two sites of phosphorylation. Third, phosphotyrosine is present in the 50,000-dalton phosphoprotein that coprecipitates with pp60src extracted from transformed chicken cells. We infer from these observations that pp60src is a novel protein kinase and that the modification of proteins via the phosphorylation of tyrosine is essential to the malignant transformation of cells by Rous sarcoma virus. pp60sarc, the closely related cellular homologue of viral pp60src, is present in all vertebrate cells. This normal cellular protein, obtained from both chicken and human cells, also phosphorylated tyrosine when assayed in an immunoprecipitate. This is additional evidence of the functional similarity of these structurally related proteins and demonstrates that all uninfected vertebrate cells contain at least one protein kinase that phosphorylates tyrosine.

Biochemical and immunological characterization of polyproteins coded for by the McDonough, Gardner-Arnstein, and Snyder-Theilen strains of feline sarcoma virus

The McDonough (SM), Gardner-Arnstein (GA), and Snyder-Theilen (ST) strains of feline sarcoma virus (FeSV) code for high-molecular-weight polyproteins that contain varying amounts of the amino-terminal region of the FeLV gag gene-coded precursor protein and a polypeptide(s) of an as yet undetermined nature. The SM-FeSV primary translational product is a 180,000-dalton polyprotein which is immediately processed into a highly unstable 60,000-dalton molecule containing the p15-p12-p30 fragment of the FeLV gag gene-coded precursor protein and a 120,000-dalton FeSV-specific polypeptide. The GA- and ST-FeSV genomes code for polyproteins of 95,000 and 85,000 daltons, respectively, which in addition to the amino-terminal moiety (p15-12 and a portion of p30) of the FeLV gag gene-coded precursor protein also contain FeSV-specific polypeptides. However, the GA- and ST-FeSV polyproteins appear to be relatively stable molecules (half-lives of around 16 h) and are not significantly processed into smaller polypeptides. Immunological and biochemical analysis of each of the above FeSV translational products revealed that the sarcoma-specific regions of the GA- and ST-FeSV polyproteins are antigenically cross-reactive and exhibit common methionine-containing peptides. These findings favor the concept that these sarcoma-specific polypeptides are coded for by the similar subsets of cellular sequences incorporated into the GA- and ST-FeSV genomes during the generation of these transforming agents.

Recombinants between endogenous and exogenous avian tumor viruses: role of the C region and other portions of the genome in the control of replication and transformation

Endogenous retroviruses of chickens are closely related to exogenous viruses isolated from spontaneous tumors in the same species, yet differ in a number of important characteristics, including the ability to transform cells in culture, ability to cause sarcomas or leukemias, host range, and growth rate in cell culture. To correlate these differences with specific sequence differences between the two viral genomes, the genome RNA of transforming subgroup E recombinants between the Prague strain of Rous sarcoma virus, subgroup B (Pr-RSV-B), and the endogenous Rous-associated virus-0 (RAV-0), Subgroup E, and seven nontransforming subgroup E recombinants between the transformation-defective mutant of Pr-RSV-B and RAV-0 was examined by oligonucleotide fingerprinting. The pattern of inheritance among the recombinant viruses of regions of the genome in which Pr-RSV-B and RAV-0 differ allowed us to draw the following conclusions. (i) Nonselected parts of the genome were, with a few exceptions, inherited by the recombinant virus progeny randomly from either parent, with no obvious linkage between neighboring sequences. (ii) A small region in the Pr-RSV-B genome which maps in the 5' region was found in all transforming but only some of the nontransforming recombinants, suggesting that it plays a role in the control of the expression of transformation. (iii) A region of the Pr-RSV-B genome which maps between env and src was similarly linked to the src gene and may be either part of the structural gene for src or a control sequence regulating the expression of src. (iv) The C region at the extreme 3' end of the virus genome which is closely related in all the exogenous avian retroviruses but distinctly different in the endogenous viruses is the major determinant responsible for the differences in growth rate between RAV-0 and Pr-RSV-B. This latter observation allowed us to redefine the C region as a genetic locus, c, with two alleles cn (in RAV-0) and cx (in exogenous viruses).

Temperature-sensitive transformation by Rous sarcoma virus and temperature-sensitive protein kinase activity

The transforming protein of Rous sarcoma virus, p60src, has associated with it a protein kinase activity. We examined whether a correlation exists between the cellular concentration of enzymatically active p60src and the degree to which chick cells are transformed by mutants of Rous sarcoma virus which are temperature-sensitive for transformation. Such a correlation does exist, but cells infected with some mutants could be shown to contain, at the nonpermissive temperature, an amount of protein kinase activity equal to 30 to 40% of that in a wild-type transformed cell. We quantified the amount of virus-induced protein kinase activity by precipitation of p60src with an excess of antitumor antiserum. Our initial measurements of activity were serious underestimates, due to the lability of the protein kinase activity associated with p60src of at least four temperature-sensitive mutants. In fact, no activity at all was associated with p60src of tsLA90 when immunoprecipitation was performed by standard means. However, when immunoprecipitation was performed with procedures which minimize inactivation, it became apparent both that cells transformed by tsLA90 contained protein kinase activity and that cells infected with either NY68 or BK5 contained at the nonpermissive temperature, one-third to one-half as much activity as wild-type transformed cells. This level of activity was much more than that arising from p60sarc in uninfected cells. In uninfected cells we found an amount of protein kinase activity which varied from 3 to 5% as much as that in a virally transformed cell. The lability of the protein kinase activity of each of these mutants is a further demonstration that this activity is essential for the transformation of cells by Rous sarcoma virus. So as to explain the high protein kinase levels in cells infected with NY68 and BK5 at the nonpermissive temperature, the idea that transformation may be a response to a small quantitative change in the total activity of p60src and the possibility that there may be more than one viral function which is essential for transformation are discussed.

Mutant avian erythroblastosis virus with restricted target cell specificity

Avian erythroblastosis virus (AEV) induces a fatal erythroblastosis within 2 weeks of intravenous injection in chicks in virtually 100% of cases. In chicks injected intramuscularly, sarcomas frequently develop at the site of injection before the animals die from erythroblastosis. In vitro, AEV transforms both erythroblasts, derived from bone marrow cultures, and fibroblasts. These effects have been shown to be a general property of AEV and not of separate leukaemia- and sarcoma-inducing forms of the virus. AEV is defective for replication and can be propagated only in the prewence of helper virus. Its transformation specificity is independent of the helper virus used. It is not clear whether AEV has two different genes controlling transformation of the two types of target cell or whether it has only one gene coding for both. To investigate this question, we looked for mutants of AEV unable to transform one of the two types of target cell. We now describe such a mutant, which is defective for erythroblast transformation but which can still transform fibroblasts.

gag-Related polypeptides encoded by replication-defective avian oncoviruses

The content of viral structural (gag) protein sequences in polypeptides encoded by replication-defective avian erythroblastosis virus (AEV) and myelocytomatosis virus MC29 was assessed by immunological and peptide analyses. Direct comparison with gag proteins of the associated helper viruses revealed that MC29 110K polypeptide contained p19, p12, and p27, whereas the AEV 75K polypeptide had sequences related only to p19 and p12. Both of these polypeptides contained some information that was unrelated to gag, pol, or env gene products. In addition, no homology was detected between these unique peptides of MC29 110K and AEV 75K. The AEV 75K polypeptide shared strain-specific tryptic peptides with the p19 encoded by its naturally occurring helper virus; this observation suggests that gag-related sequences in 75K were originally derived from the helper viral gag gene. Digestion of oxidized MC29 110K and AEV 75K proteins with the Staphylococcus aureus V8 protease generated a fragment which comigrated with N-acetylmethionylsulfoneglutamic acid, a blocked dipeptide which is the putative amino-terminal sequence of structural protein p19 and gag precursor Pr76gag. This last finding is evidence that the gag sequences are located at the N-terminal end of the MC29 110K and AEV 75K polypeptides.

Analysis of the src gene of sarcoma viruses generated by recombination between transformation-defective mutants and quail cellular sequences

Tumors were produced in quails about 2 months after injection with a transformation-defective mutant of the Schmidt-Ruppin strain of Rous sarcoma virus, subgroup A (SR-A), that retains a small portion of the src gene. Sarcoma viruses were isolated from each of five such tumors. A transformation-defective mutant which has a nearly complete deletion of the src gene was unable to induce tumors. The avian sarcoma viruses recovered from quail tumors (rASV-Q) had biological properties similar to those of the avian sarcoma viruses previously acquired from chicken tumors (rASV-C); these chicken tumors had been induced by the same transformation-defective mutants. Both rASV-Q and rASV-C transformed cells in culture with similar focus morphology and produced tumors within 7 to 14 days after injection into chickens or quails. The size of rASV-Q genomic RNA was indistinguishable from that of SR-A by polyacrylamide gel electrophoresis. The sequences of rASV-Q RNA genomes were analyzed and compared with those of the parental transformation-defective virus, SR-A and of rASV-C by RNase T1 fingerprinting and oligonucleotide mapping. We found that the src sequences of all five isolates of rASV-Q were identical to each other but different from those of SR-A and rASV-C. Of 13 oligonucleotides of rASV-Q identified as src specific, two were not found in either SR-A or rASV-C RNA. Furthermore, some oligonucleotides present in SR-A or rASV-C or both were absent in rASV-Q. No differences were found for the sequences outside the src region in any of the viruses examined. In addition, rASV-Q-infected cells possessed a 60,000-dalton protein specifically precipitable by rabbit serum raised against SR-D-induced tumors. The facts that the src sequences are essentially the same for rASV's recovered from one animal species and different for rASV's obtained from different species provide conclusive evidence that cellular sequences of normal birds were inserted into the viral genome and supplied to the resulting recombinant viruses genetic information for cell transformation.

Identification of a Rous sarcoma virus transformation-related protein in normal avian and mammalian cells

Avian sarcoma viruses (ASV) contain a gene (src) whose protein product mediates sarcomagenic transformation. This product is a 60,000-Mr phosphoprotein designated pp60src. We have found that normal uninfected frog, chicken, rat, and human cells contain a 60,000-Mr phosphoprotein related to the product of the ASV src gene and have designated that protein pp60. A phosphoprotein of similar size was not detectable in Drosophila cells. The pp60 proteins were detected by immunoprecipitation with rabbit antitumor serum containing broad spectrum antibodies to pp60src. Peptide maps of [35S]methionine-labeled pp60 and pp60src indicated major similarities as well as some differences in amino acid composition. Peptide maps of the 32P-labeled proteins demonstrated that the phosphopeptides of all endogenous pp60 molecules tested were identical. However, some differences were noted between the phosphopeptide patterns of pp60 and viral pp60src. The kinase activity associated with pp60src was measured in the immunocomplex and resulted in the transfer of radioactive phosphorus from [gamma-32P]ATP to the immunoglobulin heavy chain as well as to an 80,000-Mr phosphoprotein. The pp60 of chicken, rat, and human origin also contained an associated kinase activity. These results are consistent with the notion that the pp60 molecules are the protein products of endogenous sarc sequences found in vertebrate cells.

DNA and RNA from uninfected vertebrate cells contain nucleotide sequences related to the putative transforming gene of avian myelocytomatosis virus

The avian carcinoma virus MC29 (MC29V) contains a sequence of approximately 1,500 nucleotides which may represent a gene responsible for tumorigenesis by MC29V. We present evidence that MC29V has acquired this nucleotide sequence from the DNA of its host. The host sequence which has been incorporated by MC29V is transcribed into RNA in uninfected chicken cells and thus probably encodes a cellular gene. We have prepared radioactive DNA complementary to the putative MC29V transforming gene (cDNA(mc) (29)) and have found that sequences homologous to cDNA(mc) (29) are present in the genomes of several uninfected vertebrate species. The DNA of chicken, the natural host for MC29V, contains at least 90% of the sequences represented by cDNA(mc) (29). DNAs from other animals show significant but decreasing amounts of complementarity to cDNA(mc) (29) in accordance with their evolutionary divergence from chickens; the thermal stabilities of duplexes formed between cDNA(mc) (29) and avian DNAs also reflect phylogenetic divergence. Sequences complementary to cDNA(mc) (29) are transcribed into approximately 10 copies per cell of polyadenylated RNA in uninfected chicken fibroblasts. Thus, the vertebrate homolog of cDNA(mc) (29) may be a gene which has been conserved throughout vertebrate evolution and which served as a progenitor for the putative transforming gene of MC29V. Recent experiments suggest that the putative transforming gene of avian erythroblastosis virus, like that of MC29V, may have arisen by incorporation of a host gene (Stehelin et al., personal communication). These findings for avian erythroblastosis virus and MC29V closely parallel previous results, suggesting a host origin for src (D. H. Spector, B. Baker, H. E. Varmus, and J. M. Bishop, Cell 13:381-386, 1978; D. H. Spector, K. Smith, T. Padgett, P. McCombe, D. Roulland-Dussoix, C. Moscovici, H. E. Varmus, and J. M. Bishop, Cell 13:371-379, 1978; D. H. Spector, H. E. Varmus, and J. M. Bishop, Proc. Natl. Acad. Sci. U.S.A. 75:4102-4106, 1978; D. Stehelin, H. E. Varmus, J. M. Bishop, and P. K. Vogt, Nature [London] 260:170-173, 1976), the gene responsible for tumorigenesis by avian sarcoma virus. Avian sarcoma virus, avian erythroblastosis virus, and MC29V, however, induce distinctly different spectra of tumors within their host. The putative transforming genes of these viruses share no detectable homology, although sequences homologous to all three types of putative transforming genes occur and are highly conserved in the genomes of several vertebrate species. These data suggest that evolution of oncogenic retroviruses has frequently involved a mechanism whereby incorporation and perhaps modification of different host genes provides each virus with the ability to induce its characteristic tumors.

A normal cell protein similar in structure and function to the avian sarcoma virus transforming gene product

This report extends our previous studies concerning the identification and characterization of a protein from normal cells that is closely related to the avian sarcoma virus (ASV) transforming gene product pp60src. This normal cellular protein, which we have found in both avian and mammalian cells and have tentatively designated pp60sarc, was detected by immunoprecipitation of radiolabeled cell extracts with serum derived from both mice and rabbits bearing ASV-induced tumors. The normal cell pp60sarc is a 60,000-dalton phosphoprotein that is structurally similar, but not identical, to viral pp60src. The phosphorylation patterns of the normal cell and viral proteins are also similar: both contain two major phosphorylated residues, a phosphoserine located on the NH2-terminal 60% of the polypeptide and a phosphothreonine present on the COOH-terminal 40% of the molecule. In addition, the normal cell pp60sarc from both chicken and mammalian cells appears to have an associated protein kinase activity analogous to that previously described for the viral pp60src. The possible roles played by the normal cell protein pp60sarc and the ASV transforming protein pp60src in normal cellular growth and neoplastic disease, respectively, are discussed.

Cellular information in the genome of recovered avian sarcoma virus directs the synthesis of transforming protein

Recovered avian sarcoma viruses, whose sarcomagenic information is largely derived from cellular sequences [Wang, L.-H., Halpern, C.C., Nadel, M. & Hanafusa, H. (1978) Proc. Natl. Acad. Sci. USA 75, 5812-5816], produce the transforming protein p60src in infected cells, in amounts comparable to the amount found in cells transformed by standard strains of avian sarcoma virus. Though displaying some virus-specific differences in electrophoretic mobility, p60srcs from these viruses are similar to those of other avian sarcoma virus strains by the criteria of (i) antigenicity, (ii) partial proteolysis mapping, and (iii) association with protein kinase activity. We also find that p60sarc, a protein present in normal cells at a low level, is associated with a protein kinase activity, and thus it too is similar by the above criteria to p60src of avian sarcoma virus. Possible causes for the pathogenicity of p60src are discussed in light of these similarities.

Lymphomas resembling lymphoid leukosis in chickens inoculated with reticuloendotheliosis virus

Chickens inoculated as embyros or at hatching with the chick syncytial strain of reticuloendotheliosis virus developed a high incidence of lymphoid neoplasms between the 17th and 43rd weeks of age, involving principally the liver and bursa of Fabricius. On the basis of organ distribution, latent period, pathology and surface IgM production, the lymphomas closely resembled those of lymphoid leukosis. One inoculated chicken developed a myxosarcoma. No tumors were observed in uninoculated controls. The tumor-bearing chickens were free of infection with Marek's disease virus and exogenous avian leukosis virus (ALV) of subgroups A, B, C or D. However, the chickens were known to express endogenous ALV genes to varying degrees.

Uninfected vertebrate cells contain a protein that is closely related to the product of the avian sarcoma virus transforming gene (src)

Neoplastic transformation of cell by avian sarcoma virus is mediated by a single viral gene (src), which encodes a phosphoprotein (pp60src) with the enzymatic activity of a protein kinase. The DNAs of vertebrate species contain a highly conserved homologue of src that is also represented in the polysomal RNA of uninfected cells and, hence, may specify a normal cellular protein. We have used antisera directed against pp60src to isolate a closely related phosphoprotein (denoted vertebrate pp60) from uninfected chicken, quail, rat, and human cells. Our data indicate that vertebrate pp60 is a homologue of pp60src, highly conserved both antigenically and chemically. Moreover, the cellular protein may possess protein kinase activity similar to that associated with pp60src. We conclude that the product of src is a slightly modified analogue of a normal cellular protein.

Characterization of some isolates of newly recovered avian sarcoma virus

We previously reported the isolation of a newly recovered avian sarcoma virus (rASV) from tumors of chickens injected with transformation-defective (td) mutants of the Schmidt-Ruppin strain of Rous sarcoma virus (SR-RSV). In this paper, we present further biological and biochemical characterization of the recovered sarcoma viruses. High titers of rASV's were generally obtained by cocultivation of tumor cells with normal chicken embryo fibroblasts or by homogenization of tumor tissues. Most rASV isolates were similar to SR-RSV, subgroup A (SR-RSV-A), in their growth characteristics and were nondefective in replication. The subgroup specificity of rASV's and the electrophoretic mobilities of their structural proteins were the same as those parental td viruses. The nondefectiveness of rASV's was further substantiated by the size of their genomic RNA, which was indistinguishable from that of SR-RSV-A and substantially larger than that of parental td RNA. Molecular hybridization using complementary DNA specific to the src gene of SR-RSV (cDNAsrc) showed that the RNAs of td mutants used in this study contained extensive deletions within the src gene (7 to 30% hybridization with cDNAsrc); the same probe hybridized up to 90% with RNA from two isolates of rASV. These data indicate that rASV has regained genetic information which had been deleted in the td mutants and strongly suggest that the generation of rASV involves a genetic interaction between td virus and host cell genetic information.

An avian oncovirus mutant (SE 21Q1b) deficient in genomic RNA: biological and biochemical characterization

We have isolated a nonconditional mutant of PR-RSV-E with unique properties. This virus (SE 21Q1b) is shed from a continuously growing culture of transformed quail cells. 21Q1b virions are unable to transform or replicate in other quail or chicken cells after exogenous infection, despite the fact that the viral particles contain normal envelope glycoproteins, internal structural proteins and RNA-dependent DNA polymerase. The lack of infectivity of 21Q1b virions is a consequence of the failure to package genomic 39S RNA. Instead, these virions contain a mixture of heterogenous-sized polyadenylated cellular RNAs and 4S RNA. Less than 1% of the encapsulated RNA is viral-specific, although in the 21Q1b-producing cells, amounts of 39S, 28S and 21S viral RNAs comparable to those in wild-type virus-infected cells are synthesized and function as mRNAs for the viral proteins. Thus 21Q1b can be considered an RNA packaging mutant. Superinfection of 21Q1b cells with either RAV-1 or PR-A leads to production of about 10% or more of the normal titer of superinfecting virus, but none of the 21Q1b genetic markers are rescued. After superinfection, the 21Q1b cells continue to synthesize 21Q1b particles containing cellular RNAs in the same amounts as before infection. Thus superinfection does not appear to "switch off" the aberrant packaging of cellular RNA, but allows packaging of the superinfecting RNA. One explanation for the phenotype of 21Q1b is that the genome is lacking a signal necessary for efficient genomic RNA packaging (but not for translation) and that the 21Q1b genome encodes a "packaging factor" with an altered specificity so that cellular RNAs are efficiently packaged. 21Q1b virions do contain RNA-dependent DNA polymerase which has normal endogenous synthetic activity. The cDNA product made in vitro from detergent-lysed 21Q1b virions hybridizes equally well to uninfected quail and 21Q1b-producing quail cell RNAs, with kinetics suggesting that the endogenous product consists of transcripts of cellular RNAs present in low amounts in the cells.

Recombination between viral and cellular sequences generates transforming sarcoma virus

A series of sarcoma viruses has been obtained from tumors induced by transformation-defective (td) mutants of the Schmidt-Ruppin strain of Rous sarcoma virus, subgroup A (SR-A). The RNA sequences of these "recovered avian sarcoma viruses" (rASVs) were compared with those of td mutants and of SR-A by oligonucleotide fingerprinting. Of six sarcoma-specific oligonucleotides present in SR-A RNA, three to six were missing in the RNAs of the four td mutants examined. All six isolates of rASV examined have regained these six oligonucleotides. In addition, most rASV RNAs have three new oligonucleotides not present in the RNA either of td mutants or of SR-A. The newly obtained oligonucleotides are located between 800 and 2600 nucleotides from the 3' end of rASV RNA, which corresponds to the src region of SR-A RNA mapped previously. Furthermore, viral RNAs of two td mutants isolated from a clone of rASV lack most src-specific oligonucleotides, including the three new ones. No differences were found among RNAs of td, SR-A, and rASV in the regions outside of src. Our results indicate that RNA sequences that rASVs have acquired from cells in the process of conversion from td virus to transforming virus are mapped within the src region and segregate with the transforming function. Some of the sequences are new and some are identical with those in SR-A RNA.

In vitro isolation of stable rat sarcoma viruses

A Sprague-Dawley (SD-1) rat embryo culture, at low passage level, released an endogenous ecotropic type C virus (SD-RaLV) and after about 20 further passages it underwent spontaneous transformation. The SD-RaLV, released from the transformed cells, did not cause rapid transformation of other rat embryo cells. However, when the transformed cells were repeatedly cocultivated with three different chemically transformed and serially transplanted rat tumor cell lines (sarcoma, carcinoma, and hepatoma), rapidly fibroblast-transforming "sarcoma" viruses (RaSV) were recovered after each attempt. RaSV was not recovered from one of these tumor cell lines before transplantation, nor could focus-forming virus be rescued from these same tumor cells by cocultivation with other cells releasing heterologous type C viruses. Foci were induced on normal rat kidney and several other rat embryo cell strains within 7-15 days and both productive and nonproductive NRK clones were derived. The productive clones were positive for rat specific p30 antigen and the RaSVs released were serially transmitted to other rat embryo cells. RaSV genome was rescued from the nonproductive clones by superinfection with SD-RaLV, wild rat type C virus, and several heterologous type C viruses. These observations appear to represent naturally occurring transformation-specific (src) genes being recovered in vitro in the form of stable "sarcoma" viruses. These viruses differ from the Kirsten and Harvey strains of murine sarcoma virus in that they apparently contain no MuLV sequences and are of purely rat origin.