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

A peptide & peptide nucleic acid synthesis technology for transporter molecules and theranostics--the SPPS

Advances in imaging diagnostics using magnetic resonance tomography (MRT), positron emission tomography (PET) and fluorescence imaging including near infrared (NIR) imaging methods are facilitated by constant improvement of the concepts of peptide synthesis. Feasible patient-specific theranostic platforms in the personalized medicine are particularly dependent on efficient and clinically applicable peptide constructs. The role of peptides in the interrelations between the structure and function of proteins is widely investigated, especially by using computer-assisted methods. Nowadays the solid phase synthesis (SPPS) chemistry emerges as a key technology and is considered as a promising methodology to design peptides for the investigation of molecular pharmacological processes at the transcriptional level. SPPS syntheses could be carried out in core facilities producing peptides for large-scale scientific implementations as presented here.

Regulation of the SRC family kinases by Csk

The non-receptor tyrosine kinase Csk serves as an indispensable negative regulator of the Src family tyrosine kinases (SFKs) by specifically phosphorylating the negative regulatory site of SFKs, thereby suppressing their oncogenic potential. Csk is primarily regulated through its SH2 domain, which is required for membrane translocation of Csk via binding to scaffold proteins such as Cbp/PAG1. The binding of scaffolds to the SH2 domain can also upregulate Csk kinase activity. These regulatory features have been elucidated by analyses of Csk structure at the atomic levels. Although Csk itself may not be mutated in human cancers, perturbation of the regulatory system consisting of Csk, Cbp/PAG1, or other scaffolds, and certain tyrosine phosphatases may explain the upregulation of SFKs frequently observed in human cancers. This review focuses on the molecular bases for the function, structure, and regulation of Csk as a unique regulatory tyrosine kinase for SFKs.

From Hen House to Bedside: Tracing Hanafusa's Legacy from Avian Leukemia Viruses to SRC to ABL and Beyond

The discovery of the Src oncogene was the first step on a long journey toward improved cancer chemotherapy. In this review, we explore Src and BCR-ABL, signal transduction, and recent advances in oncogene addiction and celebrate Hidesaboro Hanafusa and the many researchers who ushered in the age of target-directed therapy against tyrosine kinase oncoproteins.

Transgene expression facilitated by the v-src splice acceptor can impair replication kinetics and lead to genomic instability of Rous sarcoma virus-based vectors

Rous sarcoma virus (RSV) can be used for the simple generation of high-titer replication-competent retroviral (RCR) vectors. Retroviruses undergo frequent genomic recombination, however, and vectors with reduced replication kinetics are rapidly overgrown by mutant forms. Vector design is hence critical to vector efficacy. In this study, two different designs of RSV-based RCR vectors were evaluated. Vectors in which transgene expression was facilitated by the v-src splice acceptor were revealed to have greatly reduced replication kinetics and genomic stability in comparison to vectors in which transgene expression was mediated by an internal ribosome entry site in the 3' untranslated region.

Molecular biology of jaagsiekte sheep retrovirus

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma (OPA), a contagious lung cancer of sheep. Until recently, research on JSRV/OPA was hampered by the lack of a tissue culture system for the propagation of the virus. Historically, pathological samples (lung fluid) collected from sheep affected by OPA were the only source of infectious JSRV. Thus studies on the JSRV/OPA system were conducted only where field isolates of OPA cases were readily available. In the past 10 years, the deduction of the JSRV sequence (York et al. 1991; York 1992), the isolation of an infectious and oncogenic JSRV molecular clone (JSRV21) (Palmarini et al. 1999a) and the establishment of a rapid method to produce infectious virus in vitro (Palmarini et al. 1999a) sparked many studies at the molecular level that strengthened past observations and revealed new properties of this unique virus. Here, we will review the data accumulated so far on the molecular biology of JSRV using the infectious and oncogenic JSRV21 molecular clone as virus of reference.

Hyaluronan activates cell motility of v-Src-transformed cells via Ras-mitogen-activated protein kinase and phosphoinositide 3-kinase-Akt in a tumor-specific manner

We investigated the production of hyaluronan (HA) and its effect on cell motility in cells expressing the v-src mutants. Transformation of 3Y1 by v-src virtually activated HA secretion, whereas G2A v-src, a nonmyristoylated form of v-src defective in cell transformation, had no effect. In cells expressing the temperature-sensitive mutant of v-Src, HA secretion was temperature dependent. In addition, HA as small as 1 nM, on the other side, activated cell motility in a tumor-specific manner. HA treatment strongly activated the motility of v-Src-transformed 3Y1, whereas it showed no effect on 3Y1- and 3Y1-expressing G2A v-src. HA-dependent cell locomotion was strongly blocked by either expression of dominant-negative Ras or treatment with a Ras farnesyltransferase inhibitor. Similarly, both the MEK1 inhibitor and the kinase inhibitor clearly inhibited HA-dependent cell locomotion. In contrast, cells transformed with an active MEK1 did not respond to the HA. Finally, an anti-CD44-neutralizing antibody could block the activation of cell motility by HA as well as the HA-dependent phosphorylation of mitogen-activated protein kinase and Akt. Taken together, these results suggest that simultaneous activation of the Ras-mitogen-activated protein kinase pathway and the phosphoinositide 3-kinase pathway by the HA-CD44 interaction is required for the activation of HA-dependent cell locomotion in v-Src-transformed cells.

The hunting of the Src

The non-receptor tyrosine kinase Src is important for many aspects of cell physiology. The viral src gene was the first retroviral oncogene to be identified, and its cellular counterpart was the first proto-oncogene to be discovered in the vertebrate genome. Src has been important, not only as an object of study in itself, but also as an entry point into the molecular genetics of cancer.

Apparent uncoupling of oncogenicity from fibroblast transformation and apoptosis in a mutant myc gene transduced by feline leukemia virus

The T17 v-myc oncogene was transduced by feline leukemia virus in a spontaneous feline T-cell lymphosarcoma. Molecular cloning and sequencing of the v-myc gene revealed several unique mutations, including a large deletion affecting amino acids 49 to 124 and a 3-bp insertion within the basic DNA binding domain which converts Leu-362 to Phe-Arg. The T17 lymphoma cell line was found to express a truncated 50-kDa Myc protein at exceptionally high levels, while the endogenous c-myc gene was not detectably expressed. These observations suggest that the mutant Myc product expresses an oncogenic function in T cells. Further evidence that the T17 mutant gene retains oncogenic potential was provided by its detection in clonally integrated proviruses in secondary tumors induced by feline leukemia virus T17, where no reversion mutations were found in any of three tumors examined. However, the mutant T17 v-myc gene did not induce transformation in a chicken embryo fibroblast assay, in contrast to wild-type feline c-myc, which conferred higher growth rates on the chicken fibroblasts, along with altered morphology and the ability to form foci in soft agar. Chicken cells over-expressing feline c-myc died by apoptosis when cultured with low serum concentrations, while the T17 mutant had no discernible effect. These results suggest that the leukemogenic potential of Myc can be uncoupled from its ability to cause transformation in fibroblasts. A possible explanation for this apparent paradox is that developing T cells are acutely sensitive to a subset of Myc functions which are insufficient for fibroblast transformation.

Homologous interference resulting from the presence of defective particles of human immunodeficiency virus type 1

Defective particles are naturally occurring virus mutants that lack one or more genes required for viral replication. Such viruses may affect positively or negatively the symptoms of the disease. Thus, it is of great interest to measure the role played by defective particles in the process of human immunodeficiency virus (HIV) infection since accumulating evidence indicates that a great proportion of HIV genomes are defective. We used defective particles produced by two stable cellular clones (UHC-8 and UHC-18) to investigate whether they can affect replication of infectious viral particles generated by a human T-cell line transfected with a molecular HIV-1 clone. Progeny virus harvested from UHC-8 cells has no reverse transcriptase and integrase proteins, while UHC-18 has no reverse transcriptase protein. We demonstrate here that coinoculation of a T-lymphoid cell line and of peripheral blood mononuclear cells with defective and infectious particles leads to a dramatic inhibition of virus replication. Defective particles do not interfere with virus production from proviral DNA. Rather, the inhibition of reinfection events seems to be their mechanism of action. This model closely parallels the in vivo conditions and demonstrates that defective particles may limit the spread of infection and progression of the disease by reducing the yield of infectious virus.

Modulating effects of the extracellular sequence of the human insulinlike growth factor I receptor on its transforming and tumorigenic potential

We reported previously that an N-terminally truncated insulinlike growth factor I receptor (IGFR) fused to avian sarcoma virus UR2 gag p19 had a greater transforming potential than did the native IGFR, but it failed to cause tumors in vivo. To investigate whether the 36 amino acids (aa) of the IGFR extracellular (EC) sequence in the gag-IGFR fusion protein encoded by the retrovirus UIGFR have a modulatory effect on the biological and biochemical properties of the protein, four mutants, NM1, NM2, NM3, and NM4 of the EC sequence were constructed. NM1 lacks the entire 36 aa residues; NM2 lacks the N-terminal 16 aa residues (aa 870 to 885), including two potential N-linked glycosylation sites of the EC sequence; NM3 contains a deletion of the C-terminal 20 aa residues (aa 886 to 905) of the EC sequence; and NM4 contains N-to-Q substitutions at both N-linked glycosylation sites. NM1 was the strongest of the four mutants in promoting anchorage-independent growth of transfected chicken embryo fibroblasts, while NM2 and NM4 had weaker transforming potential than did the original UIGFR virus. Only NM1 and NM3 were able to induce sarcomas in chickens. The four NM mutant-transformed cells expressed the expected proteins with comparable steady-state levels. The in vitro tyrosine kinase activity of P53NM1 was about fourfold higher than that of the parental P57-75UIGFR, whereas NM2 and NM4 proteins exhibited four- to fivefold-lower kinase activities. Despite lacking the IGFR EC sequence, P53NM1 formed covalent dimers similar to those formed by the parental P57-75UIGFR. Increased phosphatidylinositol (PI) 3-kinase activity was found to be associated with the mutant IGFR proteins. Among NM4 proteins. Elevated tyrosine phosphorylation of cellular proteins of 35, 120, 140, 160, and 170 kDa was detected in all mutant IGFR-transformed cells. We conclude that the EC 36-aa sequence of IGFR in the gag-IGFR fusion protein exerts intricate modulatory effects on the protein's transforming and tumorigenic potential. The 20 aa residues immediately upstream of the transmembrane domain have an inhibitory effect on the tumorigenic potential of gag-IGFR, whereas N-linked glycosylation within the EC sequence appears to have a positive effect on the transforming potential of UIGFR. Increased in vitro kinase activity and, to a lesser extent, in vivo tyrosine phosphorylation as well as the elevated association of PI 3-kinase activity with IGFR proteins seem to be correlated with the transforming potential of IGFR mutant proteins.

Carboxyl-terminal determinants of Abelson protein important for lymphoma induction

The carboxyl-terminal region of the Abelson protein is not absolutely required for Abelson virus transformation. However, Abelson virus strains encoding proteins missing portions of this region have a reduced ability to transform lymphoid cells in vitro and in vivo. One such strain, called P90A, is unique in that P90A-injected mice almost always develop tumors containing highly oncogenic variants that encode new forms of Abelson protein. In this work, we have examined the mechanism by which these variants are generated and used the variants to identify carboxyl-terminal protein sequences important for the induction of Abelson disease. Analysis of mice injected with helper-free P90A virus stocks demonstrates that the variants are generated during viral replication in vivo, probably as a consequence of error-prone reverse transcription. The sequence of the P90A viral genome reveals that a 19-base deletion is responsible for synthesis of the truncated Abelson protein. As a consequence of this mutation, 167 carboxyl-terminal amino acids normally found in the wild-type protein have been replaced by 33 amino acids derived from an alternative reading frame. Site-directed mutants show that the combination of the deletion and the P90A carboxyl terminus is required for the generation of variants. Thus, the particular structure of the P90A protein, not the specific residues lost or gained, alters the transforming potential of the Abelson protein. Finally, the sequence of the variants encoding smaller Abelson proteins reveals that as few as 452 v-abl-encoded amino acids are required for rapid induction of Abelson disease.

Analysis of cDNAs of the proto-oncogene c-src: heterogeneity in 5' exons and possible mechanism for the genesis of the 3' end of v-src

To further characterize the gene structure of the proto-oncogene c-src and the mechanism for the genesis of the v-src sequence in Rous sarcoma virus, we have analyzed genomic and cDNA copies of the chicken c-src gene. From a cDNA library of chicken embryo fibroblasts, we isolated and sequenced several overlapping cDNA clones covering the full length of the 4-kb c-src mRNA. The cDNA sequence contains a 1.84-kb sequence downstream from the 1.6-kb pp60c-src coding region. An open reading frame of 217 amino acids, called sdr (src downstream region), was found 105 nucleotides from the termination codon for pp60c-src. Within the 3' noncoding region, a 39-bp sequence corresponding to the 3' end of the RSV v-src was detected 660 bases downstream of the pp60c-src termination codon. The presence of this sequence in the c-src mRNA exon supports a model involving an RNA intermediate during transduction of the c-src sequence. The 5' region of the c-src cDNA was determined by analyzing several cDNA clones generated by conventional cloning methods and by polymerase chain reaction. Sequences of these chicken embryo fibroblast clones plus two c-src cDNA clones isolated from a brain cDNA library show that there is considerable heterogeneity in sequences upstream from the c-src coding sequence. Within this region, which contains at least 300 nucleotides upstream of the translational initiation site in exon 2, there exist at least two exons in each cDNA which fall into five cDNA classes. Four unique 5' exon sequences, designated exons UE1, UE2, UEX, and UEY, were observed. All of them are spliced to the previously characterized c-src exons 1 and 2 with the exception of type 2 cDNA. In type 2, the exon 1 is spliced to a novel downstream exon, designated exon 1a, which maps in the region of the c-src DNA defined previously as intron 1. Exon UE1 is rich in G+C content and is mapped at 7.8 kb upstream from exon 1. This exon is also present in the two cDNA clones from the brain cDNA library. Exon UE2 is located at 8.5 kb upstream from exon 1. The precise locations of exons UEX and UEY have not been determined, but both are more than 12 kb upstream from exon 1. The existence and exon arrangements of these 5' cDNAs were further confirmed by RNase protection assays and polymerase chain reactions using specific primers. Our findings indicate that the heterogeneity in the 5' sequences of the c-src mRNAs results from differential splicing and perhaps use of distinct initiation sites. All of these RNAs have the potential of coding for pp60c-src, since their 5' exons are all eventually joined to exon 2.

Analysis of cDNAs of the proto-oncogene c-src: heterogeneity in 5' exons and possible mechanism for the genesis of the 3' end of v-src

To further characterize the gene structure of the proto-oncogene c-src and the mechanism for the genesis of the v-src sequence in Rous sarcoma virus, we have analyzed genomic and cDNA copies of the chicken c-src gene. From a cDNA library of chicken embryo fibroblasts, we isolated and sequenced several overlapping cDNA clones covering the full length of the 4-kb c-src mRNA. The cDNA sequence contains a 1.84-kb sequence downstream from the 1.6-kb pp60c-src coding region. An open reading frame of 217 amino acids, called sdr (src downstream region), was found 105 nucleotides from the termination codon for pp60c-src. Within the 3' noncoding region, a 39-bp sequence corresponding to the 3' end of the RSV v-src was detected 660 bases downstream of the pp60c-src termination codon. The presence of this sequence in the c-src mRNA exon supports a model involving an RNA intermediate during transduction of the c-src sequence. The 5' region of the c-src cDNA was determined by analyzing several cDNA clones generated by conventional cloning methods and by polymerase chain reaction. Sequences of these chicken embryo fibroblast clones plus two c-src cDNA clones isolated from a brain cDNA library show that there is considerable heterogeneity in sequences upstream from the c-src coding sequence. Within this region, which contains at least 300 nucleotides upstream of the translational initiation site in exon 2, there exist at least two exons in each cDNA which fall into five cDNA classes. Four unique 5' exon sequences, designated exons UE1, UE2, UEX, and UEY, were observed. All of them are spliced to the previously characterized c-src exons 1 and 2 with the exception of type 2 cDNA. In type 2, the exon 1 is spliced to a novel downstream exon, designated exon 1a, which maps in the region of the c-src DNA defined previously as intron 1. Exon UE1 is rich in G+C content and is mapped at 7.8 kb upstream from exon 1. This exon is also present in the two cDNA clones from the brain cDNA library. Exon UE2 is located at 8.5 kb upstream from exon 1. The precise locations of exons UEX and UEY have not been determined, but both are more than 12 kb upstream from exon 1. The existence and exon arrangements of these 5' cDNAs were further confirmed by RNase protection assays and polymerase chain reactions using specific primers. Our findings indicate that the heterogeneity in the 5' sequences of the c-src mRNAs results from differential splicing and perhaps use of distinct initiation sites. All of these RNAs have the potential of coding for pp60c-src, since their 5' exons are all eventually joined to exon 2.

Rapid reversion of a deletion mutation in Moloney murine leukemia virus by recombination with a closely related endogenous provirus

During abortive infection of mouse cells, defective retroviruses carrying deletions in essential functions can recombine with endogenous retroviral sequences to form viable, replication-competent viruses. We have examined the reversion of a mutant Moloney murine leukemia virus with a deletion in the protease domain of the pol gene after infection of NIH/3T3 cells. In this system revertants arise quickly, only 2 weeks after infection. Analysis of DNA clones of the revertant viral genomes showed that they were derived by recombination with a long sequence of gag and pol exhibiting 95% sequence identity to Moloney virus. One such cloned recombinant was fully infectious, indicating that the repertoire of viral sequences in the NIH/3T3 genome must include substantial stretches of functional viral genes. Examination of the viral DNAs very early in the infection revealed the presence of defective genomes, formed by nonhomologous crossovers between the two parental sequences. We suggest that these may serve as intermediates in the eventual formation of the viable revertant genomes.

Transduction of the cellular src gene and 3' adjacent sequences in avian sarcoma virus PR2257

When injected into chickens, a transformation-defective mutant of the Prague C strain of Rous sarcoma virus induced tumors at low incidence and after a long latency. One such tumor released a replication-defective virus designated PR2257. We molecularly cloned and sequenced the proviral DNA from quail fibroblasts transformed by PR2257. Comparison of PR2257 sequence with that of Prague C, cellular src, and 3' adjacent cellular DNA showed that the spliced version of the c-src gene and about 950 base pairs (bp) of 3'-flanking cellular DNA were transduced into PR2257. This transduction eliminated nearly all replicative genes, since the gag gene splice donor site was linked to the splice acceptor site of the src gene and, on the 3' side, recombination occurred in the end of env gene. Insertion of two extra cytosines 23 bp before and 19 bp after the c-src stop codon resulted in an extension of the coding portion up to 587 amino acids, divergence of sequences after Pro-525 and replacement of Tyr-527 by a valine residue. In addition, it appears that the 5' and 3' untranslated regions of PR2257 result from multiple recombinations between exogenous and endogenous virus genomes. Limited digestion of p66src encoded by PR2257 with Staphylococcus aureus V8 protease yielded a V2 peptide (C-terminal moiety) with an apparent molecular mass of 31 kilodaltons, consistent with the 5.7-kilodalton increase expected from the DNA sequence. The structure of PR2257 suggests that the first step in the capture of c-src gene by avian lymphomatosis viruses is the trans splicing of the viral leader mRNA to exon 1 of c-src.

Transduction of proto-src sequences in tissue culture by a molecular clone of transformation-defective Rous sarcoma virus with an internal src deletion

The sporadic appearance of nondefective (nd) Rous sarcoma virus (RSV) from cells in tissue culture infected with a molecular clone of transformation-defective RSV was examined. Southern analysis of extrachromosomal, virus-specific DNA of three independent ndRSV isolates in each case indicated restoration of an isogenic src by homologous recombination with cellular proto-src. The frequency of transduction was estimated by fluctuation analysis to vary between one transduction per 0.4 x 10(7) to 1.6 x 10(7) infected cells.

Recombinant DNA related to hepatitis B and human immunodeficiency viruses in mononuclear cells of patients with AIDS

Sixty-eight of 73 patients with human immunodeficiency virus (HIV) infection were positive when tested for the presence of hepatitis B virus (HBV)-related DNA sequences in peripheral blood mononuclear cells (PBMCs) by the dot blot method. Twenty-two of the positive DNAs were examined by Southern hybridization and all exhibited a 3.2 kb extrachromosomal DNA fragment that hybridized to HBV DNA. This DNA was isolated from agarose gels and cloned into the EcoRI site of pBR322 DNA. The cloned DNA (pHBI) hybridized to both HBV DNA and HIV cDNA; HBV DNA did not hybridize to HIV cDNA under the same conditions. The results of restriction enzyme analyses indicated that pHBI contains: 1) a large deletion of HBV sequences spanning the 3' end of the HBV surface antigen gene; 2) a small deletion near the 5' end of the HBV core antigen gene; and 3) a region of homology to a one kb central section of the HIV pol gene. These data suggest that the 3.2 kb DNA found in the PBMCs is a natural recombinant between HBV and HIV DNAs raising the possibility not only that this DNA plays a role in the pathogenesis of AIDS but also that other viral recombinant DNAs may be pathogenic in human disease.

Avian sarcoma viruses

Twelve independent isolates of avian sarcoma viruses (ASVs) can be divided into four groups according to the transforming genes harbored in the viral genomes. The first group is represented by viruses containing the transforming sequence, src, inserted in the viral genome as an independent gene; the other three groups of viruses contain transforming genes fps, yes or ros fused to various length of the truncated structural gene gag. These transforming sequences have been obtained by avian retroviruses from chicken cellular DNA by recombination. The src-containing viruses code for an independent polypeptide, p60src; and the representative fps, yes and ros-containing ASVs code for P140/130gag-fps, P90gag-yes and P68gag-ros fusion polypeptides respectively. All of these transforming proteins are associated with the tyrosine-specific protein kinase activity capable of autophosphorylation and phosphorylating certain foreign substrates. p60src and P68gag-ros are integral cellular membrane proteins and P140/130gag-fps and P90gag-yes are only loosely associated with the plasma membrane. Cells transformed by ASVs contain many newly phosphorylated proteins and in most cases have an elevated level of total phosphotyrosine. However, no definitive correlation between phosphorylation of a particular substrate and transformation has been established except that a marked increase of the tyrosine phosphorylation of a 34,000 to 37,000 dalton protein is observed in most ASV transformed cells. The kinase activity of ASV transforming proteins appears to be essential, but not sufficient for transformation. The N-terminal domain of p60src required for myristylation and membrane binding is also crucial for transformation. By contrast, the gag portion of the FSV P130gag-fps is dispensable for in vitro transformation and removal of it has only an attenuating effect on in vivo tumorigenicity. The products of cellular src, fps and yes proto-oncogenes have been identified and shown to also have tyrosine-specific protein kinase activity. The transforming potential of c-src and c-fps has been studied and shown that certain structural changes are necessary to convert them into transforming genes. Among the cellular proto-oncogenes related to the four ASV transforming genes, c-ros most likely codes for a growth factor receptor-like molecule. It is possible that the oncogene products of ASVs act through certain membrane receptor(s) or enzyme(s), such as protein kinase C, in the process of cell transformation.

Identification of endogenous retroviral sequences as potential donors for recombinational repair of mutant retroviruses: positions of crossover points

Mutants of Moloney murine leukemia virus carrying deletions in essential regions of the genome can revert after infection of mouse cells by recombination with endogenous retroviral sequences. We have identified cloned DNAs containing potential donor sequences for two such recombination events and determined the nucleotide sequences in the relevant regions. Comparison of these sequences with that of the original mutants and the revertant viruses allowed a determination of the crossover points that were used in formation of the revertants. Each crossover occurred in short stretches (17-24 bp) of perfect homology between the two parent sequences.

The mechanism of transduction of proto-oncogene c-src by avian retroviruses

Chicken c-src sequences have been transduced by avian leukosis viruses (ALV) and by partial src-deletion (td) mutants of Rous sarcoma virus in several independent events. Analyses of the recombination junctions in the genomes of src-containing viruses and the c-src DNA have shed light on the mechanism of transduction, which involves at least two steps of recombination. The initial recombination between a viral genome and the 5' region of c-src appears to occur at the DNA level. This step does not require extensive homology and can be mediated by stretches of sequences with only partial homology. The 5' recombination junction can also be formed by splicing between viral and c-src sequences. The second recombination is presumed to occur between the transducing ALV or td viral RNA and the viral-c-src hybrid RNA molecule generated from the initial recombination. This step involving recombination at the 3' ends of those molecules restores the 3' viral sequences essential for replication to the viral-c-src hybrid molecule. High frequency of c-src transduction by partial td mutants suggests that the second recombination is greatly enhanced when there is sequence homology between the transducing virus and the 3' region of c-src. Incorporation of the c-src sequences into an ALV genome results in greatly elevated expression of the gene. However, increased expression of c-src alone is insufficient to activate its transforming potential. Structural changes in c-src are necessary to convert it into a transforming gene. The changes can be as small as single nucleotide changes resulting in single amino aid substitutions at certain positions. Mutations can occur rapidly during viral replication after c-src is incorporated into the viral genome. Therefore, it is most likely that transduction of c-src by ALV is followed by subsequent mutation and selection for the sarcomagenic virus. In the case of transduction by td viruses that retain certain src sequences, joining of these sequences with the transduced c-src apparently is sufficient to activate its transforming potential.

Rous sarcoma virus mutant dlPA105 induces different transformed phenotypes in quail embryonic fibroblasts and neuroretina cells

dlPA105 is a spontaneous variant of Rous sarcoma virus, subgroup E, which carries a deletion in the N-terminal portion of the v-src gene coding sequence. This virus was isolated on the basis of its ability to induce proliferation of quiescent quail neuroretina cells. The altered v-src gene encodes a phosphoprotein of 45,000 daltons which possesses tyrosine kinase activity. DNA sequencing of the mutant v-src gene has shown that deletion extends from amino acid 33 to 126 of wild-type p60v-src. We investigated the tumorigenic and transforming properties of this mutant virus. dlPA105 induced fibrosarcomas in quails with an incidence identical to that induced by wild-type virus. Quail neuroretina cells infected with the mutant virus were morphologically transformed and formed colonies in soft agar. In contrast, dlPA105 induced only limited morphological alterations in quail fibroblasts and was defective in promoting anchorage-independent growth of these cells. Synthesis and tyrosine kinase activity of the mutant p45v-src were similar in both cell types. These data indicate that the portion of the v-src protein deleted in p45v-src is dispensable for the mitogenic and tumorigenic properties of wild-type p60v-src, whereas it is required for in vitro transformation of fibroblasts. The ability of dlPA105 to induce different transformation phenotypes in quail fibroblasts and quail neuroretina cells is a property unique to this Rous sarcoma virus mutant and provides evidence for the existence of cell-type-specific response to v-src proteins.

NH2-terminal sequences of two src proteins that cause aberrant transformation

Two isolates of recovered avian sarcoma viruses (rASVs), rASV157 and rASV1702, transform cells in culture, but have greatly reduced in vivo tumorigenicity. The src proteins of rASV157 and rASV1702 have alterations within their NH2 termini, are not myristoylated, and have an altered subcellular localization. We have molecularly cloned and determined the nucleotide sequences of the src genes of rASV157 and rASV1702. We found that their src proteins have unusual NH2 termini: the rASV157 src protein NH2 terminus consists of 30 amino acids of the env signal peptide attached to Ser-6 of the src sequence, while the rASV1702 src protein NH2 terminus consists of 45 amino acids of the env signal peptide attached to Ala-76 of the src sequence. Expression of recombinant Rous sarcoma virus constructs containing the molecularly cloned rASV src genes produced src proteins with the same properties as those of the parental viruses. Our results suggest that the NH2-terminal structures are responsible for many unusual properties of the mutant src proteins.

Genetic analysis of p60v-src domains involved in the induction of different cell transformation parameters

The expression of p60v-src in chicken cells infected with Rous sarcoma virus causes stimulation of cell proliferation, morphological alteration, and anchorage independence. PA101 and PA104 are temperature-sensitive variants encoding mutant p60v-src proteins that are partially defective in the induction of these transformation parameters. To define the structural basis for the transformation defectiveness of the p60v-src mutants, the v-src genes of PA101 and PA104 were molecularly cloned and analyzed. Amino- and carboxy-terminal coding regions of the cloned mutant genes were exchanged with the corresponding regions of cloned wild-type v-src and chicken c-src genes, reconstructed into viral DNA, and expressed in infected cells maintained at various temperatures. This analysis revealed that lesions within the tyrosine kinase domains of the two mutant proteins confer temperature sensitivity on all three transformation functions of p60v-src. An amino-terminal region of the PA101 mutant protein, which coincides with the proposed modulatory domain and appears to interact with the kinase domain, affects morphological alteration in a temperature-independent manner. Our results suggest that the function of the kinase domain is essential to all three parameters examined, whereas the amino-terminal domain is important in determining cell morphology.

Abelson murine leukemia virus variants with increased oncogenic potential

A number of strains of Abelson murine leukemia virus (A-MuLV) with various abilities to transform cells have been identified. Among these is the A-MuLV-P90 strain, a mutant derived from A-MuLV-P120 that encodes an A-MuLV protein missing sequences that are normally present at the extreme carboxy terminus of P120 (N. Rosenberg and O. N. Witte, J. Virol. 33:340-348, 1980). This virus transforms NIH 3T3 cells efficiently but does not transform a high frequency of lymphoid cells in vitro or in vivo. In this communication, we show that of the relatively few tumors induced by A-MuLV-P90 nearly all contained new variant viruses that stably expressed either larger or smaller A-MuLV proteins. Strains that expressed larger A-MuLV proteins behaved like A-MuLV-P120 in transformation assays, whereas those expressing smaller A-MuLV proteins induced a high frequency of tumors after a short latent period in vivo but failed to transform large numbers of lymphoid cells in vitro. Thus, these latter viruses separated the requirements for in vitro transformation of lymphoid cells from those for tumor induction. All of the variants differed from A-MuLV-P90 in the carboxy-terminal region of the A-MuLV protein, suggesting that sequences in this region play a key role in the ability of the virus to interact with hematopoietic cells in vivo and in vitro.

Lack of induction of neuroretinal cell proliferation by Rous sarcoma virus variants that carry the c-src gene

Expression of p60v-src of Rous sarcoma virus in cultured chicken embryo neuroretinal cells was previously shown to result in the transformation and sustained proliferation of normally quiescent cell populations. We show here that Rous sarcoma virus variants that encode p60c-src, the cellular homolog of p60v-src, lack the ability to induce morphological transformation and cell proliferation of cultured neuroretinal cells. Neuroretinal cells infected with c-src-containing viruses, however, possess no less p60 protein kinase activity assayed in the immune complex than those infected with the transformation-defective Rous sarcoma virus mutants PA101 or PA104, which do stimulate the growth of these cells.

N-terminal deletions in Rous sarcoma virus p60src: effects on tyrosine kinase and biological activities and on recombination in tissue culture with the cellular src gene

We have constructed deletions within the region of cloned Rous sarcoma virus DNA coding for the N-terminal 30 kilodaltons of p60src. Infectious virus was recovered after transfection. Deletions of amino acids 15 to 149, 15 to 169, or 149 to 169 attenuated but did not abolish transforming activity, as assayed by focus formation and anchorage-independent growth. These deletions also had only slight effects on the tyrosine kinase activity of the mutant src protein. Deletion of amino acids 169 to 264 or 15 to 264 completely abolished transforming activity, and src kinase activity was reduced at least 10-fold. However, these mutant viruses generated low levels of transforming virus by recombination with the cellular src gene. The results suggest that as well as previously identified functional domains for p60src myristylation and membrane binding (amino acids 1 to 14) and tyrosine kinase activity (amino acids 250 to 526), additional N-terminal sequences (particularly amino acids 82 to 169) can influence the transforming activity of the src protein.

Transduction of c-src coding and intron sequences by a transformation-defective deletion mutant of Rous sarcoma virus

The mechanism of cellular src (c-src) transduction by a transformation-defective deletion mutant, td109, of Rous sarcoma virus was studied by sequence analysis of the recombinational junctions in three td109-derived recovered sarcoma viruses (rASVs). Our results show that two rASVs have been generated by recombination between td109 and c-src at the region between exons 1 and 2 defined previously. Significant homology between td109 and c-src sequences was present at the sites of recombination. The viral and c-src sequence junction of the third rASV was formed by splicing a cryptic donor site at the 5' region of env of td109 to exon 1 of c-src. Various lengths of c-src internal intron 1 sequences were incorporated into all three rASV genomes, which resulted from activation of potential splice donor and acceptor sites. The incorporated intron 1 sequences were absent in the c-src mRNA, excluding its being the precursor for recombination with td109 and implying that initial recombinations most likely took place at the DNA level. A potential splice acceptor site within the incorporated intron 1 sequences in two rASVs was activated and was used for the src mRNA synthesis in infected cells. The normal env mRNA splice acceptor site was used for src mRNA synthesis for the third rASV.

Activation of the transforming potential of p60c-src by a single amino acid change

Previous work showed that overexpression of the cellular src (c-src) gene does not cause transformation of chicken cells in culture. However, viral stocks isolated from cells transfected with Rous sarcoma virus DNA containing the c-src gene in place of the viral src gene did occasionally produce foci. Virus obtained from these foci were highly transforming and appeared to arise via spontaneous mutation in the c-src-containing viral populations. The p60 proteins of the transforming mutant src viruses were found to have higher levels of in vitro tyrosine kinase activity than the levels observed with the parental viruses. In this study, we have molecularly cloned the src DNA sequences of two transforming mutant src viruses. When compared to the DNA sequence of the parental c-src viruses, the mutant viruses contain single point mutations that result in single amino acid changes in the src gene products (p60 proteins). Both amino acid changes reside in the tyrosine kinase domain of the protein. The mutation detected in one virus involves replacement of the normal Glu-378 in p60c-src by Gly, whereas the p60 of the other transforming virus has Phe instead of the normal Ile-441. Our data indicate that when p60c-src is expressed at elevated levels in a retroviral context, a single amino acid change in its primary sequence can activate the kinase activity of this protein and cause cellular transformation.

A lymphocyte-specific protein-tyrosine kinase gene is rearranged and overexpressed in the murine T cell lymphoma LSTRA

Protein-tyrosine kinases are implicated in the control of normal and neoplastic cell growth. We have used molecular cloning strategies to characterize a lymphocyte-specific protein-tyrosine kinase gene distinct from but closely related to src and yes. This gene, encoded by a genetic locus defined here as lskT, is rearranged and overexpressed in the murine T cell lymphoma LSTRA. Thus alterations in the structure or expression of this protein-tyrosine kinase gene may in some cases mediate neoplastic transformation. In addition, transcription of the normal lskT gene is restricted to cells of lymphoid origin. We infer that the lskT-encoded protein-tyrosine kinase may aid in transducing proliferative or differentiative signals unique to lymphocytes.

N-terminal deletions in Rous sarcoma virus p60src: effects on tyrosine kinase and biological activities and on recombination in tissue culture with the cellular src gene

We have constructed deletions within the region of cloned Rous sarcoma virus DNA coding for the N-terminal 30 kilodaltons of p60src. Infectious virus was recovered after transfection. Deletions of amino acids 15 to 149, 15 to 169, or 149 to 169 attenuated but did not abolish transforming activity, as assayed by focus formation and anchorage-independent growth. These deletions also had only slight effects on the tyrosine kinase activity of the mutant src protein. Deletion of amino acids 169 to 264 or 15 to 264 completely abolished transforming activity, and src kinase activity was reduced at least 10-fold. However, these mutant viruses generated low levels of transforming virus by recombination with the cellular src gene. The results suggest that as well as previously identified functional domains for p60src myristylation and membrane binding (amino acids 1 to 14) and tyrosine kinase activity (amino acids 250 to 526), additional N-terminal sequences (particularly amino acids 82 to 169) can influence the transforming activity of the src protein.

Lack of induction of neuroretinal cell proliferation by Rous sarcoma virus variants that carry the c-src gene

Expression of p60v-src of Rous sarcoma virus in cultured chicken embryo neuroretinal cells was previously shown to result in the transformation and sustained proliferation of normally quiescent cell populations. We show here that Rous sarcoma virus variants that encode p60c-src, the cellular homolog of p60v-src, lack the ability to induce morphological transformation and cell proliferation of cultured neuroretinal cells. Neuroretinal cells infected with c-src-containing viruses, however, possess no less p60 protein kinase activity assayed in the immune complex than those infected with the transformation-defective Rous sarcoma virus mutants PA101 or PA104, which do stimulate the growth of these cells.

Low level of cellular protein phosphorylation by nontransforming overproduced p60c-src

We have previously found that Rous sarcoma virus variants in which the viral src (v-src) gene is replaced by the cellular src (c-src) gene have no transforming activity. In this study, we analyzed the basis for the inability of the p60c-src overproduced by these variants to transform cells. Phosphorylations of tyrosine residues in total cell protein or in cellular 34K protein are known to be markedly enhanced upon infection with wild-type Rous sarcoma virus. We found that these tyrosine phosphorylations were only slightly increased in the c-src-containing virus-infected cells, whereas both levels were significantly increased by infection with wild-type Rous sarcoma virus, or transforming mutant viruses which are derived from c-src-containing viruses by spontaneous mutation. Phosphorylation at tyrosine 416 of p60 itself was also extremely low in overproduced p60c-src and high in p60s of transforming mutant viruses. In immunoprecipitates with monoclonal antibody, the overproduced p60c-src had much lower casein tyrosine kinase activity than did p60v-src. We previously showed that p60 myristylation and plasma membrane localization may be required for cell transformation. p60c-src was similar to transforming p60s in these properties. These results strongly suggest that the low level of tyrosine phosphorylation by overproduced p60c-src accounts for its inability to transform cells.

Homologous recombination between a defective virus and a chromosomal sequence in mammalian cells

Replacement of the early region of simian virus 40 results in virus that cannot replicate in a normal host, CV-1 cells, but can replicate in COS cells, a derivative of CV-1 cells that constitutively express simian virus 40 tumor antigen (T antigen). However, passage of such an early replacement simian virus 40 mutant in COS cells results in the emergence of virus that can propagate in CV-1 cells. Analysis of this virus revealed that the mutant rescued the integrated T-antigen gene from the COS cell genome. Comparison of the sequence of the recovered virus with that of the viral DNA resident in COS cells (strain 776) and the mutant used in our studies (derived from strain 777) proves that the mutant virus acquired the T-antigen gene from the COS cell chromosome via homologous recombination. Most probably this process was mediated by a direct genetic exchange.

Low level of cellular protein phosphorylation by nontransforming overproduced p60c-src

We have previously found that Rous sarcoma virus variants in which the viral src (v-src) gene is replaced by the cellular src (c-src) gene have no transforming activity. In this study, we analyzed the basis for the inability of the p60c-src overproduced by these variants to transform cells. Phosphorylations of tyrosine residues in total cell protein or in cellular 34K protein are known to be markedly enhanced upon infection with wild-type Rous sarcoma virus. We found that these tyrosine phosphorylations were only slightly increased in the c-src-containing virus-infected cells, whereas both levels were significantly increased by infection with wild-type Rous sarcoma virus, or transforming mutant viruses which are derived from c-src-containing viruses by spontaneous mutation. Phosphorylation at tyrosine 416 of p60 itself was also extremely low in overproduced p60c-src and high in p60s of transforming mutant viruses. In immunoprecipitates with monoclonal antibody, the overproduced p60c-src had much lower casein tyrosine kinase activity than did p60v-src. We previously showed that p60 myristylation and plasma membrane localization may be required for cell transformation. p60c-src was similar to transforming p60s in these properties. These results strongly suggest that the low level of tyrosine phosphorylation by overproduced p60c-src accounts for its inability to transform cells.

Fine structural mapping of a critical NH2-terminal region of p60src

We have recently demonstrated that an NH2-terminal sequence required for myristylation and membrane association of the Rous sarcoma virus transforming protein, p60src, is contained within amino acids 2-14 [Cross, F.R., Garber, E. A., Pellman, D. & Hanafusa, H. (1984) Mol. Cell. Biol. 4, 1834-1842]. This sequence is also required for cell transformation. We have now constructed five mutants of Rous sarcoma virus that contain alterations in the src sequence coding for these 14 amino acids. Mutants encoding src proteins with a peptide insertion between amino acids 1 and 2, or peptide substitutions for amino acids 2-4, 3-4, or 7-15, were transformation-defective. The src proteins of these mutants differed from the wild-type protein in that they were not myristylated and did not fractionate with the plasma membrane of infected cells. The fifth mutant encoded a src protein with a short peptide substituted for amino acids 11-15. This protein was myristylated and plasma membrane associated, and the virus transformed cells. We therefore conclude that a sequence required for myristylation and membrane association of p60src is located within the first 7-10 amino acids of the src protein, and that p60src myristylation and membrane association are required for cell transformation. Consistent with this idea, we have isolated four transforming revertants from one of the transformation-defective mutants. The src proteins of all four revertants were found to be myristylated and membrane associated.

Recombination between a defective retrovirus and homologous sequences in host DNA: reversion by patch repair

The genomes of mammalian species contain multiple copies of sequences homologous to exogenous retroviruses. When a mutant retrovirus carrying a lethal deletion in an essential viral gene was introduced into mammalian cells, revertant viruses appeared and spread throughout the culture. Analysis of one such revertant showed that the mutation had been repaired by homologous recombination with endogenous sequences. Our results suggest that defective retroviruses can draw upon the genetic complement of the host cell to repair lesions in viral genes.

A short sequence in the p60src N terminus is required for p60src myristylation and membrane association and for cell transformation

We have constructed mutants by using linker insertion followed by deletion in the region of cloned Rous sarcoma virus DNA coding for the N-terminal 9 kilodaltons of the src protein. Previous work implicated this region in the membrane association of the protein. The mutations had little effect on src tyrosine kinase activity. Substitution of a tri- or tetrapeptide for amino acids 15 to 27, 15 to 49, or 15 to 81 had little effect on the in vitro transforming capacity of the virus. Like wild-type p60src, the src proteins of these mutants associated with plasma membranes and were labeled with [3H]myristic acid. In contrast, a mutant whose src protein had the dipeptide Asp-Leu substituted for amino acids 2 to 81 and a mutant with the tripeptide Asp-Leu-Gly substituted for amino acids 2 to 15 were transformation defective, and the mutant proteins did not associate with membranes and were not labeled with [3H]myristic acid. These results suggest that amino acids 2 to 15 serve as an attachment site for myristic acid and as a membrane anchor. Since deletions including this region prevent transformation, and since tyrosine kinase activity is not diminished by the deletions, these results imply that target recognition is impaired by mutations altering the very N terminus, perhaps through their effect on membrane association.

Increased transcription of the c-myc oncogene in two methylcholanthrene-induced quail fibroblastic cell lines

The expression of three c-onc genes (c-erb, c-myc, c-myb) was investigated in five cell lines established from fibrosarcomas induced with 20-methylcholanthrene (MCA) of Japanese quails. These cell lines showed low levels of the three c-onc genes, with the exception of two cell lines that accumulated moderate (MCAQ 1-4) and large amounts (MCAQ3-5) of c-myc RNA. Molecular cloning and restriction endonuclease analyses indicated that expression of c-myc in these two cell lines were not associated with detectable rearrangements in the c-myc locus, that the size of the c-myc transcript (2.7 kb) in MCAQ 3-5 was similar to that of the normal c-myc messenger RNAs (mRNA) and that the transcriptional activation observed in MCAQ 3-5 was not mediated by the LTR (long terminal repeat) of a proximate ALV (avian leukosis virus) provirus. Finally, when analysed with the restriction enzymes Msp I and Hpa II, the c-myc locus of MCAQ 3-5 and MCAQ 1-4 was found hypomethylated as compared with that of the other cell lines tested that show low levels of c-myc transcripts. Our results suggest that one of the ways methylcholantrene could mediate transformation is by inducing an abnormal regulation of the c-myc gene.

Mechanisms for the generation of src-deletion mutants and recovered sarcoma viruses: identification of viral sequences involved in src deletions and in recombination with c-src sequences

The precise src deletions in six transformation-defective (td) deletion mutants derived from the Schmidt-Ruppin strain of Rous sarcoma virus were determined by sequence analysis. Examination of the parental viral sequences neighboring the junctions of deletions in these td mutants revealed that these regions contained either directly repeated or inverted complementary sequences ranging from 9 to 28 nucleotides. Five td mutants were found to contain deletions flanked by directly repeated sequences, of which the 3' direct repeat was retained whereas the 5' direct repeat was deleted in the resulting td viral RNA. In the deletions of two td mutants where inverted complementary sequences were present at junctions of the deletions, both copies of the inverted complementary sequence were deleted in the td viral RNA. It is proposed from these observations that deletions of these mutants have been generated during the synthesis of minus-strand viral DNA by reverse transcriptase by jumping over a sequence flanked by direct repeats or by skipping a stem-and-loop structure formed via inverted complementary sequences on the viral RNA template. Data provide further information on the sequences in the td viral genome that are required for the generation of recovered sarcoma viruses (rASVs) by recombination with c-src. Sequence data of td viruses revealed that retaining as few as 82 nucleotides of the 3' src coding sequence is sufficient, whereas retaining as much as one-third of the 5' src but none of the 3' src coding sequences is not sufficient, for the generation of rASVs. Those that generate replication-competent rASVs retain, in addition to the 3' src region, a portion of the 5' src and/or its immediate upstream sequence that is homologous to exon 1 of the c-src DNA. These two sequence domains apparently provided 5' and 3' homologous regions for recombination between td viral genome and c-src DNA resulting in nondefective rASVs. Td109, which was shown previously to generate only replication-defective rASVs, retains 296 nucleotides of the 3' src sequence but lacks all the 5' src and 316 nucleotides of its immediate upstream region. It is concluded that the 5' src coding sequence and its immediate upstream region are not essential for the generation of rASVs. However, retaining a portion of those sequences is required for the generation of replication-competent rASVs.

A short sequence in the p60src N terminus is required for p60src myristylation and membrane association and for cell transformation

We have constructed mutants by using linker insertion followed by deletion in the region of cloned Rous sarcoma virus DNA coding for the N-terminal 9 kilodaltons of the src protein. Previous work implicated this region in the membrane association of the protein. The mutations had little effect on src tyrosine kinase activity. Substitution of a tri- or tetrapeptide for amino acids 15 to 27, 15 to 49, or 15 to 81 had little effect on the in vitro transforming capacity of the virus. Like wild-type p60src, the src proteins of these mutants associated with plasma membranes and were labeled with [3H]myristic acid. In contrast, a mutant whose src protein had the dipeptide Asp-Leu substituted for amino acids 2 to 81 and a mutant with the tripeptide Asp-Leu-Gly substituted for amino acids 2 to 15 were transformation defective, and the mutant proteins did not associate with membranes and were not labeled with [3H]myristic acid. These results suggest that amino acids 2 to 15 serve as an attachment site for myristic acid and as a membrane anchor. Since deletions including this region prevent transformation, and since tyrosine kinase activity is not diminished by the deletions, these results imply that target recognition is impaired by mutations altering the very N terminus, perhaps through their effect on membrane association.

Characterization of two strains of avian sarcoma virus isolated from avian lymphatic leukosis virus-induced sarcomas

Two replication-defective avian sarcoma viruses, S1 and S2, which were independently isolated from tumors of chickens inoculated with avian lymphatic leukosis virus (LLV) were characterized. The genomes of S1 and S2 contain src-related sequences and are, respectively, about 3.9 and 4.5 kilobases long. pp60src-related proteins with molecular weights of 62,000 (p62) were detected in cells infected with these viruses, and protein kinase activity was found to be associated with these proteins. No other viral proteins, such as gag, pol, and env gene products, were detected. These results suggested that the c-src sequence in normal chicken cells was incorporated into LLV genomes by recombination at the expense of most of the viral genes to generate highly defective new sarcoma viruses.

Rous sarcoma virus variants that carry the cellular src gene instead of the viral src gene cannot transform chicken embryo fibroblasts

The transforming activity of the cellular src (c-src) gene as well as of hybrid genes between viral and cellular src was tested by constructing derivatives of Rous sarcoma virus DNA in which all or part of the viral src gene (v-src) was replaced by the corresponding portion of the c-src gene. After these derivatives were introduced into chicken embryo fibroblasts by transfection, replication-competent virus was recovered, which induced the expression of p60src at a level equivalent to p60v-src expression in cells infected with Rous sarcoma virus wild type. Replacement of the portion of the v-src gene, either upstream or downstream of the Bgl I site, with the homologous portion of the c-src gene resulted in fully transforming viruses. On the other hand, the virus stock obtained from cells transfected with Rous sarcoma virus DNA containing the entire c-src gene had a very low titer of focus-forming virus, while it contained a high titer of infectious virus. We present evidence that the rare small foci are formed by mutant viruses generated from the original c-src-containing virus. These results indicate that overproduction of the c-src gene product does not cause cell transformation, and that this proto-oncogene is subject to a relatively high rate of mutation when incorporated in a retrovirus genome, resulting in the acquisition of transforming capacity.

Induction of tumors and generation of recovered sarcoma viruses by, and mapping of deletions in, two molecularly cloned src deletion mutants

td108 , a transformation-defective (td) deletion mutant of the Schmidt-Ruppin strain of Rous sarcoma virus of subgroup A (SR-A), was molecularly cloned. Two isolates of td viruses, td108 -3b and td108 -4a, obtained by transfection of the molecularly cloned td108 DNAs into chicken embryo fibroblasts, were tested for their ability to induce tumors and generate recovered avian sarcoma viruses ( rASVs ) in chickens. Both td viruses were able to induce tumors with a latency and frequency similar to those observed previously with biologically purified td mutants of SR-A. rASVs were isolated from most of the tumors examined. The genomic RNAs of those newly obtained rASVs were analyzed by RNase T1 oligonucleotide fingerprinting. The results showed that they had regained the deleted src sequences and contained the same set of marker src oligonucleotides as those of rASVs analyzed previously. The src oligonucleotides of rASVs are distinguishable from those present in SR-A. We conclude that those rASVs must have been generated by recombination between the molecularly cloned td mutants and the c-src sequence. The deletions in the td mutants were mapped by restriction enzyme analysis and nucleotide sequencing. td108 -3b was found to contain an internal src deletion of 1,416 nucleotides and to retain 57 and 105 nucleotides of the 5' and 3' src coding sequences, respectively. td108 -4a contained a src deletion of 1,174 nucleotides and retained 180 and 225 nucleotides of the 5' and 3' src sequences, respectively. Comparison of sequences in the 5' src and its upstream region of td108 -3b with those of SR-A, rASV1441 (a td108 -derived rASV analyzed previously), and c-src suggested that the 5' recombination between td108 and c-src occurred from 7 to 20 nucleotides upstream from the beginning of the src coding sequence.