OK10, an avian acute leukemia virus of the MC29 subgroup with a unique genetic structure

The RNA of defective avian acute leukemia virus OK10 was isolated from a defective virus particle, released by OK10-transformed nonproducer avian fibroblasts, as a 60S complex consisting of 8.6-kilobase subunits. Oligonucleotide fingerprinting and RNA.cDNA hybridization identified two sets of sequences in OK10 RNA: group-specific sequences, which are related to all nondefective members of the avian tumor virus group, and a sequence closely related to the subgroup-specific sequences (mcv) of the myelocytomatosis virus (MC29) subgroup of avian acute leukemia viruses. Hence, OK10 is classified as a member of the MC29 subgroup of avian tumor viruses, in agreement with classification based on its oncogenic spectrum. The group-specific sequences of OK10 RNA include partial (Delta) pol and env genes, a c-region, and, unlike those of all other members of the MC29 subgroup, a complete gag gene. Oligonucleotide mapping revealed 5'-gag-Deltapol-mcv-Deltaenv-c-3' as the order of the subgroup-specific and group-specific elements of OK10 RNA. The genetic unit gag-Deltapol-mcv, measuring approximately 6.4 kilobases, codes for the nonstructural, presumably transforming, 200,000-dalton OK10-specific protein and also includes the gag gene coding for the internal virion proteins. Because gag is the only intact virion gene shared in addition to regulatory RNA sequences between OK10 and nondefective avian tumor viruses, it is concluded that the gag gene is sufficient for the formation of a defective virus particle. Comparisons among the RNAs and gene products of different viruses of the MC29 subgroup show that they share 5'-terminal gag-related and internal mcv sequences but differ from each other in intervening gag-, pol-, and mcv-related sequences. It follows that the probable transforming genes and their protein products have two essential domains, one consisting of conserved 5' gag-related and the other of 3' mcv-related sequence elements. In the light of this and previous knowledge we can now distinguish two designs among five different transforming onc genes of avian tumor viruses: onc genes with coding sequences unrelated to virion genes, like those of Rous sarcoma virus and avian myeloblastosis virus, and onc genes with coding sequences that are hybrids of virion genes and specific sequences, like those of the MC29 subgroup viruses, of avian erythroblastosis virus, and of Fujinami sarcoma virus.     

Structure and specific sequences of avian erythroblastosis virus RNA: evidence for multiple classes of transforming genes among avian tumor viruses.

Two major RNA species were found in several clonal isolates of avian erythroblastosis virus (AEV) and avian erythroblastosis-associated helper virus (AEAV) complexes: one of 8.7 kilobases (kb), the other of 5.5 kb. The 5.5-kb species was identified as AEV RNA because (i) it was absent from non-transforming AEAV isolated from the same virus complex, (ii) it was present in complexes of AEV and different helper viruses, and (iii) its structure is similar to that of avian acute leukemia viruses of the MC29 group. Molecular hybridization indicated that 54% of AEV RNA is specific and 46% is related to other viruses of the avian tumor virus group, particularly to AEAV, therefore termed group-specific. The genetic structure of AEV RNA was deduced by mapping oligonucleotides representing specific and group-specific sequences and by comparing the resulting map to maps of AEAV and of other avian tumor viruses derived previously. AEV RNA contains a gag gene-related, 5' group-specific section of 1 kb, an internal AEV-specific section of 3 kb unrelated to any other viral RNA tested, and a 3' group-specific section of 1.5 kb. The 5' section of AEV RNA is closely related to analogous 5' sections of the MC29 group viruses and is homologous with a 5' RNA section that is part of the gag gene of AEAV. The 3' section is also shared with AEAV RNA and includes a variant C-oligonucleotide near the 3' end that is different from the highly conserved counterparts of all other exogenous avian tumor viruses. By analogy with Rous sarcoma virus and the acute leukemia viruses of the MC29 group, the internal specific section of AEV RNA is thought to signal a third class of onc genes in avian tumor viruses. Comparisons with AEAV and the MC29 group viruses suggest that both the 5' gag-related and the internal specific RNA sections of AEV are necessary for onc gene function.     

Genetic structure of avian myeloblastosis virus, released from transformed myeloblasts as a defective virus particle

Chicken myeloblasts transformed by avian myeloblastosis virus (AMV) in the absence of nondefective helper virus (termed nonproducer cells) were found to release a defective virus particle (DVP) that contains avian tumor viral gag proteins but lacks envelope glycoprotein and a DNA polymerase. Nonproducer cells contain a Pr76 gag precursor protein and also a protein that is indistinguishable from the Pr180 gag-pol protein of nondefective viruses. The RNA of the DVP is 7.5 kilobases (kb) long and is 0.7 kb shorter than the 8.2-kb RNAs of the helper viruses of AMV, MAV-1 and MAV-2. Comparisons based on RNA.cDNA hybridization and mapping of RNase T1-resistant oligonucleotides indicated that DVP RNA shares with MAV RNAs nearly isogenic 5'-terminal gag and pol-related sequences of 5.3 kb and a 3'-terminal c-region of 0.7 kb that is different from that found in other avian tumor viruses. Adjacent to the c-region, DVP RNA contains a contiguous specific sequence of 1.5 kb defined by 14 specific oligonucleotides. Except for two of these oligonucleotides that map at its 5' end, this sequence is unrelated to any sequences of nondefective avian tumor viruses of four different envelope subgroups as well as to the specific sequences of fibroblast-transforming avian acute leukemia and sarcoma viruses of four different RNA subgroups. The specific sequence of the DVP RNA is present in infectious stocks of AMV from this and other laboratories in an AMV-transformed myeloblast line from another laboratory, and it is about 70% related to nucleotide sequences of E26 virus, an independent isolate of an AMV-like virus. Preliminary experiments show DVP to be leukemogenic if fused into susceptible cells in the presence of helper virus. We conclude that DVP RNA is the leukemogenic component of infectious AMV and that its specific sequence, termed AMV, may carry genetic information for oncogenicity. Thus we have found here a transformation-specific RNA sequence, unrelated to helper virus, in a highly oncogenic virus that does not transform fibroblasts.     

Homology exists among the transforming sequences of avian and feline sarcoma viruses.

Fujinami sarcoma virus (FSV) of chickens does not contain nucleotide sequences related to the src gene of Rous sarcoma virus, but it carries unique sequences of at least 3000 bases, which are likely to code for the transforming protein of this virus. Using radioactive DNA complementary to FSV-unique sequences, we investigated the relatedness of FSV to other sarcoma-leukemia retroviruses in vertebrates. Under conditions of moderate stringency, no cross-hybridization was detected between FSV cDNA and RNAs of Rous sarcoma virus, Y73 avian sarcoma virus, several representative avian acute leukemia viruses, or Abelson murine leukemia virus. This cDNA, however, did hybridize with RNA of PRCII sarcoma virus of chickens to the extent of 56%. In addition, FSV cDNA was found to hybridize with RNAs of Gardner-Arnstein and Snyder-Theilen strains of feline sarcoma virus to the extent of 27% and 19%, respectively, but not with RNA of McDonough feline sarcoma virus. Studies on thermal denaturation of hybrids showed that the melting temperatures of the heteroduplexes of the FSV cDNA with RNAs of PRCII and Gardner-Arnstein feline sarcoma virus were 7 degrees C and 12 degrees C lower, respectively, compared with the melting temperature of the homologous hybrid of FSV, and suggested less than 10% mismatching in both heteroduplexes. These results indicate that nucleotide sequences closely related to at least a part of FSV-unique sequences are present in the genomes of other sarcoma viruses obtained in chickens and in cats.     

Isolation of 16L virus: a rapidly transforming sarcoma virus from an avian leukosis virus-induced sarcoma.

We have isolated a replication-defective rapidly transforming sarcoma virus (designated 16L virus) from a fibro-sarcoma in a chicken infected with td107A, a transformation-defective deletion mutant of subgroup A Schmidt-Ruppin Rous sarcoma virus. 16L virus transforms fibroblasts and causes sarcomas in infected chickens within 2 wk. Its genomic RNA is 6.0 kilobases and contains sequences homologous to the transforming gene (fps) of Fujinami sarcoma virus (FSV). RNase T1 oligonucleotide analysis shows that the 5' and 3' terminal sequences of 16L virus are indistinguishable from (and presumably derived from) td107A RNA. The central part of 16L viral RNA consists of fps-related sequences. These oligonucleotides fall into four classes: (i) oligonucleotides common to the putative transforming regions of FSV and another fps-containing avian sarcoma virus, UR1; (ii) an oligonucleotide also present in FSV but not in UR1; (iii) an oligonucleotide also present in UR1 but not in FSV; and (iv) an oligonucleotide not present in either FSV, UR1, or td107A. Cells infected with 16L virus synthesize a protein of Mr 142,000 that is immunoprecipitated with anti-gag antiserum. This protein has protein kinase activity. These results suggest that 16L virus arose by recombination between td107A and the cellular fps gene.     

The RNA of avian acute leukemia virus MC29.

The RNA of myelocytoma virus MC29, a replication-defective avian acute leukemia virus, was investigated. Sedimentation and electrophoretic analyses indicated that the virus contains a distinct 28S RNA with about 5700 nucleotides. It is the smallest avian tumor virus RNA detected to date. The small size of the RNA suggests that the defectiveness of the virus is due to deletions in replicative genes. The RNA shared 3 to 5 of 30 large RNase T1-resistant oligonucleotides with the RNA of other avian leukosis and sarcoma and may represent the transforming information of the virus. Sequences of the conserved transforming gene of avian sarcoma viruses were not detected in MC29 RNA define a new class of avian tumor viral transforming genes.     

Nucleotide sequence of avian carcinoma virus MH2: two potential onc genes, one related to avian virus MC29 and the other related to murine sarcoma virus 3611.

The 5.2-kilobase (kb) RNA genome of avian carcinoma virus MH2 has the genetic structure 5'-delta gag (0.2 kb)- mht (1.2 kb)-myc (1.4 kb)-c (0.4 kb)-poly(A) (0.2 kb)-3'. delta gag is a partial retroviral core protein gene, mht and myc are cell-derived MH2-specific sequences, and c is the 3'-terminal retroviral vector sequence. Here we have determined the nucleotide sequence of 3.5 kb from the 3' end of delta gag to the 3' end of molecularly cloned proviral MH2 DNA, in order to elucidate the genetic structure of the virus and to compare it with other mht - and myc-containing oncogenic viruses as well as with the chicken proto-myc gene. The following results were obtained: (i) delta gag- mht forms a hybrid gene with a contiguous reading frame of 2682 nucleotides that terminates with a stop codon near the 3' end of mht . The 3' 969 nucleotides of mht up to the stop codon are 80% sequence related to the onc-specific raf sequence of murine sarcoma virus 3611 (94% homologous at the deduced amino acid level). (ii) The myc sequence is preceded by an RNA splice acceptor site shared with the cellular proto-myc gene, beyond which it is colinear up to a 3'-termination codon and 40 noncoding nucleotides with the myc sequences of avian retrovirus MC29 and chicken proto-myc. Thus, myc forms, together with a 5' retroviral exon, a second MH2-specific gene. (iii) myc is followed by the 3'-terminal c region of about 400 nucleotides, which is colinear with that of Rous sarcoma virus except for a substitution near the 5' end of the long terminal repeat. It is concluded that MH2 contains two genes with oncogenic potential, the delta gag- mht gene, which is closely related to the delta gag-raf transforming gene of MSV 3611, and the myc gene, which is related to the transforming gene of MC29. Furthermore, it may be concluded that the cellular proto-onc genes, which on sequence transduction become viral onc genes, are a small group because among the 19 known onc sequences, 5 are shared by different taxonomic groups of viruses of which the mht /raf homology is the closest determined so far.     

Comparative tryptic peptide mapping studies suggest a role in cell transformation for the gag-related protein of avian erythroblastosis virus and avian myelocytomatosis virus strains CMII and MC29.

The gag-related proteins found in cells transformed by avian erythroblastosis virus (AEV) and the avian myelocytomatosis viruses MC29 and CMII have been compared by tryptic peptide fingerprinting. A comparison of the methionine-containing tryptic peptides of the AEV 75-kilodalton protein, the CMII 90-kilodalton protein, and the MC29 110-kilodalton protein with the gag gene product Pr76 of their naturally occurring helper leukemia viruses enabled us to distinguish those peptides related to the gag gene from the non-gag-related peptides. The 12 non-gag peptides found in the AEV 75-kilodalton protein were unique to this protein and not found in the MC29 110-kilodalton or CMII 90-kilodalton proteins. In contrast, the MC29 110-kilodalton protein shared two methionine-containing non-gag tryptic peptides with the CMII 90-kilodalton protein. When these experiments were repeated with [14C]lysine and [14C]arginine as the labeled amino acids, the MC29 110-kilodalton protein and the CMII 90-kilodalton protein were found to share 30 out of approximately 40 non-gag-related peptides. These results demonstrate that viruses with a similar transformation spectrum synthesize related proteins and suggest that the gag-related proteins represent the transforming proteins of the replication-defective avian leukemia viruses.     

Genome of avian myelocytomatosis virus MC29: analysis by heteroduplex mapping.

The virion RNA of avian myelocytoma virus MC29 was hybridized to full genome length DNA of the Prague strain of Rous sarcoma virus and analyzed by heteroduplex mapping in the electron microscopy. The results show that MC29 specific sequences for which there are no homologous counterparts in the Rous sarcoma virus genome make up a contiguous stretch of RNA about 1.8 kilobases long. These sequences are located approximately in the middle of the genome, replacing the 3' half of the gag gene, the entire pol gene, and the 5' portion of the env gene, which are absent from MC29. This MC29 specific genetic substitution may contain information for the leukemogenic transformation of the host cell.     

Characterization of the transforming gene of Fujinami sarcoma virus.

The src gene present in all avian sarcoma viruses is not present in the genome of Fujinami sarcoma virus, a potent sarcoma-inducing virus in chickens. Fujinami virus is defective and requires helper virus for replication. RNA from a mixture of helper and transforming viruses consists of two components, 35S and 28S. Oligonucleotide fingerprinting of each RNA component revealed that the 35S component was identical to the RNA of the helper virus. Thus, the genome of Fujinami virus must be the 28S RNA, which corresponds approximately to a molecular weight of 1.7 x 10(6) or 5300 nucleotides. Fujinami viral RNA shares several oligonucleotides with helper viral RNA at both 3' and 5' ends but contains a unique sequence of at least 3000 nucleotides in the middle of the genome. Fujinami viral RNA contains no src-specific oligonucleotides of the Rous sarcoma virus genome and did not hybridize with DNA complementary to the src sequences. The 60,000-dalton src protein of Rous sarcoma virus was undetectable in Fujinami virus-transformed cells. Instead, these transformed cells contain a protein of 140,000 daltons precipitable by antisera against virion proteins, which is likely to be the transforming protein of this virus.     

Avian acute leukemia viruses MC29 and MH2 share specific RNA sequences: Evidence for a second class of transforming genes

The genome of the defective avian tumor virus MH2 was identified as a RNA of 5.7 kilobases by its presence in different MH2-helper virus complexes and its absence from pure helper virus, by its unique fingerprint pattern of RNase T1-resistant (T1) oligonucleotides that differed from those of two helper virus RNAs, and by its structural analogy to the RNA of MC29, another avian acute leukemia virus. Two sets of sequences were distinguished in MH2 RNA: 66% hybridized with DNA complementary to helper-independent avian tumor viruses, termed group-specific, and 34% were specific. The percentage of specific sequences is considered a minimal estimate because the MH2 RNA used was about 30% contaminated by helper virus RNA. No sequences related to the transforming src gene of avian sarcoma viruses were found in MH2. MH2 shared three large T1 oligonucleotides with MC29, two of which could also be isolated from a RNase A- and T1-resistant hybrid formed between MH2 RNA and MC29 specific cDNA. These oligonucleotides belong to a group of six that define the specific segment of MC29 RNA described previously. The group-specific sequences of MH2 and MC29 RNA shared only the two smallest out of about 20 T1 oligonucleotides associated with MH2 RNA. It is concluded that the specific sequences of MH2 and MC29 are related, and it is proposed that they are necessary for, or identical with, the onc genes of these viruses. These sequences would define a related class of transforming genes in avian tumor viruses that differs from the src genes of avian sarcoma viruses.     

Cleavage of four avian sarcoma virus polyproteins with virion protease p15 removes gag sequences and yields large fragments that function as tyrosine phosphoacceptors in vitro.

The transformation-specific polyproteins of avian sarcoma viruses PRCII, PRCII-p, Fujinami sarcoma virus (FSV), and Esh sarcoma virus (ESV) consist of two domains, one derived from a partial viral gag gene and the other representing an apparently cell-derived insert in the defective viral genome. These gag-linked proteins were cleaved with retrovirion protease p15. Cleavage of PRCII-p polyprotein P170, P105 of PRCII, and P140 of FSV occurred within the gag domain and generated fragments of Mr 130,000, 70,000, and 115,000, respectively, containing all of the transformation-specific sequences linked to a remnant of the original gag sequences. ESV P80 was cleaved inside the transformation-specific domain, yielding a Mr 35,000--38,000 fragment from the NH2-terminal half of the molecule consisting of the entire gag portion and some no-gag sequences and a Mr 48,000 fragment containing most of the transformation-specific sequences. The tyrosine phosphorylation sites of the polyproteins were found in every case in the transformation-specific fragments. The major serine phosphorylation site of ESV P80 was found to reside in the Mr 35,000--38,000 gag-containing fragment, probably within the transformation-specific sequences of that cleavage product. Removal of all of the gag domain of ESV P80 or most of the gag domain in PRCII-p P170, PRCII P105, and FSV P140 does not affect their ability to be phosphorylated by the polyprotein-associated tyrosine-specific protein kinase activities. This observation suggests that the gag sequences of the polyproteins of classes II (PRCII-p, PRCII, and FSV) and III (ESV) avian sarcoma viruses may not be required for this enzymatic function, which appears to be of importance in transformation.     

A third class of avian sarcoma viruses, defined by related transformation-specific proteins of Yamaguchi 73 and Esh sarcoma viruses

The gag-linked transformation-specific protein (polyprotein) p80 of Esh avian sarcoma virus (ESV) has been compared by tryptic peptide mapping with the homologous protein p90 of Yamaguchi 73 avian sarcoma virus (Y73). p80 of ESV and p90 of Y73 were found to share all four of their major nonstructural, transformation-specific, methionine-containing peptides and to have at least seven cysteine-containing transformation-specific peptides in common. Two nonstructural cysteine-containing peptides unique for ESV p80 and three specific for Y73 p90 were also identified. None of these peptides were found in the transforming gene product pp60src of Rous sarcoma virus (RSV) or in the transformation-specific polyproteins p105 of avian sarcoma virus PRCII (PRCII) or p140 of Fujinami sarcoma virus (FSV). ESV p80 and Y73 p90 are phosphorylated, and their tryptic phosphopeptides appear to be identical. In each polyprotein two major phosphopeptides were demonstrated, one containing phosphoserine, the other phosphotyrosine. The latter serves as phosphoacceptor for the protein kinase activities (ATP:protein phosphotransferase, EC 2.7.1.37) associated with p80 and p90. These protein kinase activities were found to be functionally indistinguishable but could be easily distinguished from the activities associated with PRCII p105 and FSV p140 on the basis of their cation requirement and target site specificity. On that basis also, p80/p90-associated protein kinases were found to be more similar to the enzymatic activity of pp60src than to those associated with the PRCII and FSV transformation-specific polyproteins. These results document a close genetic relationship between the two independently isolated sarcoma viruses Y73 and ESV. On the basis of the relatedness of transformation-specific proteins, ESV and Y73 constitute class III of avian sarcoma viruses, with class I containing the various strains of RSV and class II encompassing FSV and PRCII.     

Separation of sarcoma virus-specific and leukemia virus-specific genetic sequences of Moloney sarcoma virus.

We have studied the nucleic acid sequences in nonproducer cells transformed by Moloney sarcoma virus or Abelson leukemia virus (two types of replication-defective, RNA-containing, viruses isolated by passage of Moloney leukemia virus in BALB/c mice). DNA probes from the Moloney leukemia in virus detect RNA in both Abelson virus-transformed nonproducer cells and Moloney sarcoma virus-transformed nonproducer cells. A sarcoma-specific cDNA, prepared from the Moloney sarcoma virus, has extensive homology to RNA found in heterologous nonproducer cells transformed by Moloney sarcoma virus, has little homology to RNA in cells producing Moloney leukemia virus, and no detectable homology to RNA in nonproducer cells transformed by the Abelson virus. By analogy to earlier data on avian and mammalian sarcoma viruses, these results suggest that the Moloney sarcoma virus arose by recombination between a portion of the Moloney leukemia virus genome and additional sarcoma-specific information, and indicate that the expression of this information in not essential for Abelson virus-mediated fibroblast transformation.     

pp60v-src tyrosine kinase is expressed and active in sarcoma-free avian embryos microinjected with Rous sarcoma virus.

Early embryonic avian tissue is resistant to transformation by Rous sarcoma virus. To determine the nature of this resistance, we examined the expression and properties of the Rous sarcoma virus transforming protein pp60v-src, in infected embryonic chicken limbs in ovo. Lysates from Rous sarcoma virus-infected limbs contained the viral structural protein p19gag, as detected by immunoblot analysis, and showed pp60v-src kinase activity in vitro. Immunoblot analysis of lysates with anti-phosphotyrosine antibodies revealed a number of phosphotyrosine-containing proteins present in lysates of Rous sarcoma virus-infected embryos but not in lysates of control, uninfected embryos. Anti-phosphotyrosine immunoreactivity was observed in frozen sections in the same cell types that expressed pp60v-src and p19gag. These studies demonstrate that pp60v-src is co-expressed with viral structural determinants in infected embryonic avian tissue. Furthermore, pp60v-src is active in ovo as a tyrosine-specific phosphotransferase, despite the apparent lack of sarcoma induction. The localization pattern of the major src gene substrate p36 (calpactin I) was compared with that of p19gag by double-label immunofluorescence and found to be generally nonoverlapping. These observations are consistent with the concept that the induction of tumors in ovo requires complementation between viral determinants and host factors. These host factors, which may be critical substrates of pp60v-src, are subject to developmental regulation in the avian embryo.     

Avian carcinoma virus MH2 contains a transformation-specific sequence, mht, and shares the myc sequence with MC29, CMII, and OK10 viruses.

Avian carcinoma virus MH2 has been grouped together with MC29, CMII, and OK10, because all of these viruses share a transformation-specific sequence termed myc. A 5.2-kilobase (kb) DNA provirus of MH2 has been molecularly cloned. The complete genetic structure of MH2 is 5''-delta gag(1.9-kb)-mht(1.2-kb)-myc(1.3-kb)-delta env(?) and noncoding c-region (0.2-kb)-3''. delta gag, delta env, and c are genetic elements shared with nondefective retroviruses, whereas mht is a unique, possibly MH2 transformation-specific, sequence. Hybridizations with normal chicken DNA and cloned chicken c-myc DNA indicate that the mht sequence probably derives from a normal cellular gene that is distinct from the c-myc gene. The genetic structure of MH2 suggests that the delta gag and mht sequences function as a hybrid gene that encodes the p100 putative transforming protein. The myc sequence of MH2 appears to encode a second transforming function. Therefore, it seems that MH2 contains two genes with possible oncogenic function, whereas MC29, CMII, and OK10 each carries a single hybrid delta gag-myc transforming gene. It is remarkable that, despite these fundamental differences in their primary structures and mechanisms of gene expression, MH2 and MC29 have very similar oncogenic properties.     

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.     

Pheasant virus: new class of ribodeoxyvirus.

Cocultivation of cells derived from embryos of golden pheasants or Amherst pheasants with chicken embryo cells infected with Bryan strain of Rous sarcoma virus resulted in the detection of viruses which appear to be endogenous in these pheasant cells. The pheasant viruses (PV) were similar to avian leukosis-sarcoma viruses (ALSV) in their gross morphology, in the size of their RNA, in the presence of a virion-associated RNA-dependent DNA polymerase (DNA nucleotidyltransferase; deoxynucleoside triphosphate: DNA deoxynucleotidyltransferase; EC 2.7.7.7), and in their growth characteristics. PV also serves as a helper for the glycoprotein-defective Rous sarcoma virus. However, PV was shown to be different from both ALSV and reticuloendotheliosis virus in the following properties: (i) PV does not have ALSV group specific antigens; (ii) the protein composition of PV is different from those of the other two groups of viruses; (iii) PV fails to complement the defective polymerase of alpha type Rous sarcoma virus; and (iv) PV RNA shows no detectable homology with nucleic acids of the other two groups of viruses. Thus, PV appears to be a new class of RNA viruses which contain RNA-dependent DNA polymerase.     

Acute leukemia viruses E26 and avian myeloblastosis virus have related transformation-specific RNA sequences but different genetic structures, gene products, and oncogenic properties

Replication-defective acute leukemia viruses E26 and myeloblastosis virus (AMV) cause distinct leukemias although they belong to the same subgroup of oncogenic avian tumor viruses based on shared transformation-specific (onc) RNA sequences. E26 causes predominantly erythroblastosis in chicken and in quail, whereas AMV induces a myeloid leukemia. However, upon cultivation in vitro for >1 month, a majority of surviving hemopoietic cells of E26-infected animals bear myeloid markers similar to those of AMV-transformed cells. We have analyzed the genetic structure and gene products of E26 virus for a comparison with those of AMV. An E26/helper virus complex was found to contain two RNA species: a 5.7-kilobase (kb) RNA that hybridizes with cloned AMV-specific proviral DNA and hence is probably the E26 genome; and an 8.5-kb RNA that is unrelated to AMV and represents helper virus RNA. Thus, E26 RNA is smaller than 7.5-kb AMV RNA. Hybridization of size-selected poly(A)-terminating E26 RNA fragments with AMV-specific DNA indicated that the shared specific sequences are located in the 5′ half of the E26 genome as opposed to a 3′ location in AMV RNA. In nonproducer cells transformed in vitro by E26, a gag-related nonstructural 135,000-dalton protein (p135) was found. No gag(Pr76) or gag-pol (Pr180) precursors of essential virion proteins, which are present in AMV nonproducer cells, were observed. p135 was also found in cultured E26 virus producing cells of several leukemic chickens, and its intracellular concentration relative to that of the essential virion proteins encoded by the helper virus correlates with the ratio of E26 to helper RNA in virions released by these cells. p135 is phosphorylated but not glycosylated; antigenically it is not related to the pol or env gene products. It appears to be coded for by a partial gag gene and by E26-specific RNA sequences, presumably including those shared with AMV. Hence, AMV and E26 appear to use different strategies for the expression of related onc sequences: AMV is thought to encode a transforming protein via a subgenomic mRNA, whereas E26 codes for a gag-related polyprotein via genomic RNA. It is speculated that differences in the oncogenic properties of E26 and AMV are due to differences in their genetic structures and gene products.