Infection of chick cells by subgroup E viruses

Identification and characterization of avian retroviruses in chicken embryo-derived yellow fever vaccines: investigation of transmission to vaccine recipients

All currently licensed yellow fever (YF) vaccines are propagated in chicken embryos. Recent studies of chick cell-derived measles and mumps vaccines show evidence of two types of retrovirus particles, the endogenous avian retrovirus (EAV) and the endogenous avian leukosis virus (ALV-E), which originate from the chicken embryonic fibroblast substrates. In this study, we investigated substrate-derived avian retrovirus contamination in YF vaccines currently produced by three manufacturers (YF-vax [Connaught Laboratories], Stamaril [Aventis], and YF-FIOCRUZ [FIOCRUZ-Bio-Manguinhos]). Testing for reverse transcriptase (RT) activity was not possible because of assay inhibition. However, Western blot analysis of virus pellets with anti-ALV RT antiserum detected three distinct RT proteins in all vaccines, indicating that more than one source is responsible for the RTs present in the vaccines. PCR analysis of both chicken substrate DNA and particle-associated RNA from the YF vaccines showed no evidence of the long terminal repeat sequences of exogenous ALV subgroups A to D in any of the vaccines. In contrast, both ALV-E and EAV particle-associated RNA were detected at equivalent titers in each vaccine by RT-PCR. Quantitative real-time RT-PCR revealed 61,600, 348,000, and 1,665,000 ALV-E RNA copies per dose of Stamaril, YF-FIOCRUZ, and YF-vax vaccines, respectively. ev locus-specific PCR testing of the vaccine-associated chicken substrate DNA was positive both for the nondefective ev-12 locus in two vaccines and for the defective ev-1 locus in all three vaccines. Both intact and ev-1 pol sequences were also identified in the particle-associated RNA. To investigate the risks of transmission, serum samples from 43 YF vaccine recipients were studied. None of the samples were seropositive by an ALV-E-based Western blot assay or had detectable EAV or ALV-E RNA sequences by RT-PCR. YF vaccines produced by the three manufacturers all have particles containing EAV genomes and various levels of defective or nondefective ALV-E sequences. The absence of evidence of infection with ALV-E or EAV in 43 YF vaccine recipients suggests low risks for transmission of these viruses, further supporting the safety of these vaccines.

Characterization of endogenous avian leukosis viruses in chicken embryonic fibroblast substrates used in production of measles and mumps vaccines

Previous findings of low levels of reverse transcriptase (RT) activity in chick cell-derived measles and mumps vaccines showed this activity to be associated with virus particles containing RNA of both subgroup E endogenous avian leukosis viruses (ALV-E) and endogenous avian viruses (EAV). These particles originate from chicken embryonic fibroblast (CEF) substrates used for propagating vaccine strains. To better characterize vaccine-associated ALV-E, we examined the endogenous ALV proviruses (ev loci) present in a White Leghorn CEF substrate pool by restriction fragment length polymorphism. Five ev loci were detected, ev-1, ev-3, ev-6, ev-18, andev-19. Both ev-18 and ev-19 can express infectious ALV-E, while ev-1, ev-3, and ev-6 are defective. We analyzed the full-length sequence of ev-1 and identified an adenosine insertion within the pol RT-beta region at position 5026, which results in a truncated RT-beta and integrase. We defined the 1,692-bp deletion in the gag-pol region of ev-3, and we found that in ev-6, sequences from the 5' long terminal repeat to the 5' pol region were absent. Based on the sequences of the ev loci, RT-PCR assays were developed to examine expression of ALV-E particles (EV) in CEF supernatants. Both ev-1- and ev-3-like RNA sequences were identified, as well as two other RNA sequences with intact pol regions, presumably of ev-18 and ev-19 origin. Inoculation of susceptible quail fibroblasts with CEF culture supernatants from both 5-azacytidine-induced and noninduced CEF led to ALV infection, confirming the presence of infectious ALV-E. Our data demonstrate that both defective and nondefective ev loci can be present in CEF vaccine substrates and suggest that both ev classes may contribute to the ALV present in vaccines.

Molecular cloning and characterization of the RNA packaging-defective retrovirus SE21Q1b

The nonconditional RNA packaging mutant SE21Q1b contains cis- and trans-acting defects which cause cellular mRNA, rather than viral genomic RNA, to be nonspecifically packaged into SE21Q1b viral particles. Using genomic libraries of the c-SE21Q1b quail cell line, we have been able to construct a molecular clone of the SE21Q1b provirus. Upon transfection into primary quail embryo fibroblasts, the SE21Q1b molecular clone is able to recapitulate the nonspecific RNA packaging phenotype of the c-SE21Q1b cell line. The RNA packaging phenotypes displayed by several SE21Q1b/avian sarcoma-leukemia virus hybrid provirus constructs have further indicated that sequences responsible for the altered RNA packaging phenotype of SE21Q1b are localized in the left third of the SE21Q1b proviral genome. DNA sequence analysis of this region has revealed that the 5' SE21Q1b deletion has removed 179 bp from the SE21Q1b left long terminal repeat and leader regions. Several differences were detected at the carboxyl terminus of the deduced SE21Q1b nucleocapsid protein sequence in comparison with that of Rous sarcoma virus PR-C. Results of site-directed oligonucleotide mutagenesis experiments indicate, however, that the presence of these residues in the nucleocapsid protein alone is not responsible for the decreased RNA packaging specificity of SE21Q1b.

Role of RAV-0 genes in the permissive replication of subgroup E avian leukosis viruses on line 15Bev1 CEF

Rous associated virus type-0 (RAV-0) is a replication-competent endogenous virus of chickens which grows more efficiently on chick embryo fibroblasts (CEFs) from line 15B chickens than on CEFs from line K28. Differences in viral growth on these two sources of cells have been attributed to an early event in the retrovirus life-cycle, at or before viral DNA synthesis. Five in vitro constructed avian leukosis viruses (ALVs) as well as RAV-0 (a subgroup E ALV), RAV-1 (subgroup A), and RAV-2 (subgroup B) have been assessed for their relative growth on 15Bev1 and K28 CEFs. More efficient replication on 15Bev1 CEFs than on K28 CEFs was determined by subgroup E-encoding sequences in env. Subgroup A and B envelope sequences as well as viral LTR, gag, and pol sequences did not obviously bias relative rates of viral replication on the two cell types. We suggest that the unusually permissive replication of subgroup E viruses on 15B CEFs is a receptor-mediated phenomenon and that the line 15B receptor for subgroup E ALVs is more efficient than that of line K28.

Sequence comparison in the crossover region of an oncogenic avian retrovirus recombinant and its nononcogenic parent: genetic regions that control growth rate and oncogenic potential

NTRE 7 is an avian retrovirus recombinant of the endogenous nononcogenic Rous-associated virus-0 (RAV-0) and the oncogenic, exogenous, transformation-defective (td) Prague strain of Rous sarcoma virus B (td-PrRSV-B). Oligonucleotide mapping had shown that the recombinant virus is indistinguishable from its RAV-0 parent except for the 3'-end sequences, which were derived from td-PrRSV-B. However, the virus exhibits properties which are typical of an exogenous virus: it grows to high titers in tissue culture, and it is oncogenic in vivo. To accurately define the genetic region responsible for these properties, we determined the nucleotide sequences of the recombinant and its RAV-0 parent by using molecular clones of their DNA. These were compared with sequences already available for PrRSV-C, a virus closely related to the exogenous parent td-PrRSV-B. The results suggested that the crossover event which generated NTRE 7 took place in a region -501 to -401 nucleotides from the 3' end of the td-PrRSV parental genome and that sequences to the right of the recombination region were responsible for its growth properties and oncogenic potential. These sequences included a 148-base-pair exogenous-virus-specific region that was absent from the RAV-0 genome and the U3 region of the long terminal repeat. Since the exogenous-virus-specific sequences are expected to be missing from transformation-defective mutants of the Schmidt-Ruppin strain of RSV, which, like other exogenous viruses, grow to high titers in tissue culture and are oncogenic in vivo, we concluded that the growth properties and oncogenic potential of the exogenous viruses are determined by sequences in the U3 region of the long terminal repeat. However, we propose that the exogenous-virus-specific region may play a role in determining the oncogenic spectrum of a given oncogenic virus.

Genomes of endogenous and exogenous avian retroviruses

The endogenous viruses of chickens are closely related to the exogenous avian leukosis viruses (ALV) yet as a group differ from these viruses in their host range, growth rate, and oncogenicity. The present study was undertaken to determine the patterns of relationship among the genomes of endogenous and exogenous ALVs. Complete or partial T1 oligonucleotide maps were prepared from the genomes of endogenous viruses that reside at eight distinct loci in chickens. Selected endogenous viruses and recombinants of endogenous or endogenous and exogenous viruses were characterized for host range and growth rate. From these data we could infer the following: (1) Endogenous viruses form a distinct lineage of ALVs with the most distinctive differences occurring in the portion of env that encodes host range and the U3 portion of the long terminal repeat; (2) The U3 sequences of endogenous ALVs determine the low growth rates of these viruses; and (3) Endogenous ALVs have distinctive oligonucleotide markers that allow them to be subclassified into distinct lineages. Our results suggest that endogenous viruses are derived from one another and not from exogenous field strains of ALV. This phenomenon may be related to the unique env encoded host range of endogenous ALVs, their unique U3 encoded growth rates, or perhaps their unique access, as residents of germ line DNA, to germ line cells.

Sequence comparison in the crossover region of an oncogenic avian retrovirus recombinant and its nononcogenic parent: genetic regions that control growth rate and oncogenic potential

NTRE 7 is an avian retrovirus recombinant of the endogenous nononcogenic Rous-associated virus-0 (RAV-0) and the oncogenic, exogenous, transformation-defective (td) Prague strain of Rous sarcoma virus B (td-PrRSV-B). Oligonucleotide mapping had shown that the recombinant virus is indistinguishable from its RAV-0 parent except for the 3'-end sequences, which were derived from td-PrRSV-B. However, the virus exhibits properties which are typical of an exogenous virus: it grows to high titers in tissue culture, and it is oncogenic in vivo. To accurately define the genetic region responsible for these properties, we determined the nucleotide sequences of the recombinant and its RAV-0 parent by using molecular clones of their DNA. These were compared with sequences already available for PrRSV-C, a virus closely related to the exogenous parent td-PrRSV-B. The results suggested that the crossover event which generated NTRE 7 took place in a region -501 to -401 nucleotides from the 3' end of the td-PrRSV parental genome and that sequences to the right of the recombination region were responsible for its growth properties and oncogenic potential. These sequences included a 148-base-pair exogenous-virus-specific region that was absent from the RAV-0 genome and the U3 region of the long terminal repeat. Since the exogenous-virus-specific sequences are expected to be missing from transformation-defective mutants of the Schmidt-Ruppin strain of RSV, which, like other exogenous viruses, grow to high titers in tissue culture and are oncogenic in vivo, we concluded that the growth properties and oncogenic potential of the exogenous viruses are determined by sequences in the U3 region of the long terminal repeat. However, we propose that the exogenous-virus-specific region may play a role in determining the oncogenic spectrum of a given oncogenic virus.

Clonal analysis of the integration and expression of endogenous avian retroviral DNA acquired by exogenous viral infection

Rous-associated virus-0 is one of several endogenous avian retroviruses that are transmitted vertically and that can be isolated from different inbred lines of chickens. These viruses, referred to here as induced-leukosis viruses bearing a subgroup E glycoprotein (ILV-E), are all closely related. Clonal populations of fibroblasts from line 15B and line 100 inbred chickens have been examined for the presence and expression of exogenously acquired ILV-E sequences. Restriction enzyme analysis of uniform populations of line 15B fibroblasts, prepared by cloning cells either before or after infection with ILV-E, indicates that viral sequences were inserted at multiple sites within the cell genome. Analysis of 49 clonal populations of line 100 fibroblasts containing between one and five copies of exogenous ILV-E sequences demonstrated that each clone was characterized by a unique set of viral DNA insertions within the cell genome. The expression of the exogenous ILV-E sequences within these fibroblast clones was examined by using reverse transcriptase activity as a measure of virus production. Some clones produced an amount of virus equivalent to that produced by an equal number of the uncloned ILV-E-infected parental fibroblasts. Other clones produced 5- to 10-fold less virus. Still other clones produced no detectable virus at all. Among nine clones derived from cells containing a single copy of the ILV-E provirus, the level of virus production differed more than 100-fold. DNA from these clones was analyzed with several different restriction endonucleases to characterize the location and arrangement of the ILV-E sequences. All nine clones consisted of cells that appeared to contain a complete provirus inserted (i) in a different site within the cellular DNA and (ii) in an orientation that was colinear with the viral genomic RNA. It was observed that several cleavage sites potentially affected by methylation were equally available for cleavage in all clones regardless of the level of viral production.

Transfer of defective avian tumor virus genomes by a Rous sarcoma virus RNA packaging mutant

SE21Q1b, a Rous sarcoma virus mutant which packages cellular rather than viral RNA, is competent for infection of quail cells and can transmit defective transforming retrovirus genes. Stably transformed recipient clones have been obtained by using this mutant.

Integration, expression, and infectivity of exogenously acquired proviruses of Rous-associated virus-O

We investigated the integration sites, infectivities, and expression of Rous-associated virus-0 (Rav-0) DNAs in exogenously infected turkey and chicken cells. Restriction endonuclease analyses of the DNAs of RAV-0-infected cells indicated that unique integration sites of RAV-0 DNA were detectable in clones of RAV-0-infected cells but not in mass-infected cell cultures. In addition, the sites of integration of RAV-0 DNA differed in each of the seven clones of RAV-0-infected cells examined. Thus, endogenous RAV-0 proviruses appeared to integrate at multiple sites in cellular DNA, which were distinct from the sites of integration of endogenous RAV-0 genomes. Since exogenous RAV-0 proviruses are expressed at 10(3)- to 10(4)-fold-higher levels and are 10(3)- to 10(4)-fold more infectious in transfection assays than the endogenous RAV-0 genome of uninfected V+ chicken cells, these results are consistent with the hypothesis that transcription of the endogenous RAV-0 genome is regulated by flanking cellular DNA sequences. Although all RAV-0-infected cells contained infectious RAV-0 DNA and produced high titers of RAV-0 compared with uninfected V+ cells, different clones of RAV-0 infected chicken cells differed by as much as 30-fold in their levels of virus production. The infectivity of the DNA of each clone of RAV-0-infected cells correlated with the amount of virus produced by that clone. However, these differences did not appear to be correlated either with the number of exogenous RAV-0 proviruses in different clones or with the infectivity of RAV-0 produced by different clones, indicating that differences either in modification of RAV-0 DNAs or in the cellular sequences flanking exogenous RAV-0 DNAs were responsible for the observed differences in expression and infectivity.

Avian oncovirus mutant (SE21Q1b) deficient in genomic RNA: characterization of a deletion in the provirus

We have previously described a nonconditional mutant of avian sarcoma virus (SE21Q1b) which fails to package viral RNA (Gallis et al., Virology 94:146-161, 1979; Linial et al., Cell 15:1371-1381, 1978). Quail cells transformed by SE21Q1b contain normal amounts of intracellular viral mRNA's for src, env, and gag-pol and release particles with the density of normal virus containing a typical complement of virion proteins, including reverse transcriptase. These virions are noninfectious for both chicken and quail cells and contain primarily cellular rather than viral RNA. Analysis by gel electrophoresis of the cellular DNA of quail cells transformed by SE21Q1b after restriction endonuclease digestion indicated the presence of a single provirus. The provirus was located at one site in the genome of the host cell and was flanked by the characteristic terminally repeated sequences derived from the 3' and 5' ends of viral RNA. The only defect detected in the SE21Q1b provirus was a deletion of ca. 150 base pairs of DNA somewhere between 300 and 600 bases from the left (gag-pol) end of the provirus. Analyses of the proviral DNA of cells transformed by wild-type recombinants between SE21Q1b and leukosis viruses reveal that the recombinants no longer contain this deletion. The deletion, therefore, defines a region on the viral RNA which is required for correct packaging of the virion RNA.

Synthesis and processing of viral glycoproteins in two nonconditional mutants of Rous sarcoma virus

We have studied the pattern of glycoprotein synthesis in two nonconditional mutants of Rous sarcoma virus. One mutant, SE33, produces no viral particles but synthesizes Pr92env, which is cleaved intracellularly to mature glycoproteins. The second mutant, SE521, encodes a gPr92env which is not cleaved to gp85 or gp37 and therefore produces virions with the phenotype of Bryan RSV(-) or NY8. Neither of these mutants have detectable genomic deletions. The study of these mutants has led to the following conclusions. (i) In the absence of particle production or p15 synthesis, gPr92env can be cleaved to the mature glycoprotein which is found on the cell surface. (ii) Noncleaved gPr92env is not packaged into virions but is found on the cell surface. (iii) gPr92env alone can account for subgroup specific viral interference. (iv) gPr92env is probably transported to the cell surface before additional glycosylation or cleavage to mature virion glycoprotein. The nonprocessed precursor of SE521 appears to be glycosylated normally, and thus far we have been unable to determine the basis for the defect in this mutant.

Synthesis and processing of polymerase proteins of wild-type and mutant avian retroviruses

We have studied the biosynthesis of avian retrovirus proteins related to reverse transcriptase in permissive avian embryonic cells. Analysis of immune precipitates from avian sarcoma virus (ASV)-infected cells demonstrated the presence of the 180,000-dalton gag-pol "read-through" protein (Pr180gag-pol) and a 130,000-dalton polypeptide (Pr130gag-pol). Pr130gag-pol was found, in serological and peptide mapping studies, to consist primarily of sequences related to reverse transcriptase and the gag-encoded protein p15. Pr180gag-pol was found to be phosphorylated, whereas Pr130gag-pol was not. In addition, only Pr180gag-pol but not Pr130gag-pol was susceptible to cleavage with the virion protease p15. Although the structure of Pr130gag-pol would suggest that it is generated by removal of a portion of the gag region from Pr180gag-pol, an analysis of labeling kinetics has failed to demonstrate unequivocally whether Pr130gag-pol is a cleavage product of Pr180gag-pol or a primary translation product. We were repeatedly unable to detect either Pr180gag-pol or Pr130gag-pol in virus particles released from the cell, whereas both beta and alpha subunits were readily observed. Several presumed intermediates between Pr130gag-pol and the beta subunit of reverse transcriptase were also observed in virions. These studies indicate cleavage of polyemrase precursors at the time of virus budding. On the basis of these data, we present a processing scheme for the generation of reverse transcriptase subunits. We have also examined reverse transcriptase biosynthesis in cells producing two mutants that fail to package the enzyme. Previous work showed that integrated proviruses of both mutants are missing DNA sequences in pol: one mutant, PH9 (Mason et al., J. Virol. 30:132-140, 1979), contains a deletion near the 3' end of pol, whereas the other, SE52d (linial et al., Virology 87:130-141, 1978), may have inserted a host cell sequence near the 5' end of pol. Neither mutant synthesized Pr180gag-pol or Pr130gag-pol, but instead produced novel proteins comprised of sequences shared with gag proteins plus a region antigenically related to reverse transcriptase. Both proteins were defective as precursors to reverse transcriptase. Whereas Pr180gag-pol and Pr130gag-pol were precipitated by an antiserum raised against p32 (a virion protein derived from the portion of the beta subunit removed during processing of beta to alpha [Schiff and Grandgenett, J. Virol. 28:279-291, 1978]), the novel protein synthesized by PH9 ws not precipitated. This suggets that the alpha subunit is generated by a COOH-terminal cleavage of the beta subunit.

Induction of neoplasms by subgroup E recombinants of exogenous and endogenous avian retroviruses (Rous-associated virus type 60)

Chickens susceptible to infection with subgroup E viruses were inoculated with four independent isolates of Rous-associated virus type 60 (RAV-60) that are subgroup e recombinants of endogenous and exogenous virus. Neoplasms developed in each inoculated group. Therefore, nontransforming viruses of subgroup E can induce lymphoid leukosis at a moderate rate compared with RAV-0, a subgroup E endogenous virus, suggesting that oncogenicity is not a viral envelope (env)-related characteristic. Since the common (c) regions of the RAV-60s examined were of exogenous origin, we suggest that the c region rather than env is important for a high rate of induction of lymphoid leukosis and related neoplasms.

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

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

Mutant and recombinant avian retroviruses with extended host range

Avian retroviruses of subgroups B and D efficiently infect chicken (C/E) but not turkey (T/BD) cells. We describe here three variants of subgroup B and D viruses that infect both cell types equally well. One of these viruses, NTRE-4, was a recombinant between transformation-defective Prague (Pr) strain Rous sarcoma virus (RSV) subgroup B and the endogenous virus RAV-0; the second, SR-DE-1, was a recombinant between Schmidt-Ruppin RSV subgroup D and defective endogenous virus information. T1 oligonucleotide fingerprint analysis of the genomes of these two viruses showed only a small alteration in the portion of the env gene responsible for subgroup specificity, as indicated by the presence of a single subgroup E oligonucleotide in an otherwise purely subgroup B or D gene. The third virus, hrBO1Pr-B, was a variant of Pr-RSV-B that did not appear to be a recombinant and whose altered host range we attribute to mutation. Analysis of the host range of all three viruses by infection of selectively resistant cells and by interference testing indicates that all use the subgroup B receptor on chicken cells and the subgroup E receptor on turkey cells. These viruses may be analogous to the polytropic recombinant viruses recently found to be associated with leukemia in some strains of mice.

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

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

Regulation of expression and chromosomal subunit conformation of avian retrovirus genomes

We have investigated the copy number, chromosomal subunit conformation and regulation of expression of integrated avian retrovirus genomes. Our results indicate that there are approximately two copies of the endogenous viral genomes (RAV-O) per haploid cell genome in uninfected chick embryo fibroblasts (CEF) and red blood cells (RBC). The copy number and subunit conformation (as measured by DNAasel sensitivity) of the RAV-O genomes are independent of the level of expression of these viral DNA sequences. In cells isolated from embryos of the V+, gs-chf- and gs+chf+ phenotypes, approximately one of the two viral genomes is in a DNAase l-sensitive conformation. Upon infection with an exogenous Rous sarcoma virus (PR-RSV-C), one new viral genome is integrated per haploid CEF genome. The newly integrated RSV genome is completely sensitive to DNAase l, and the subunit conformation of the endogenous viral genomes is not altered by the integration of additional exogenous proviruses. Both the endogenous and newly integrated exogenous viral genomes are present in "nu-body" structures, and the selective sensitivity of these proviral DNA sequences to DNAase l is maintained in isolated nucleosomes. Our experiments revealing the DNAase l sensitivity of one of the two RAV-O genomes in gs-chf-CEF led us to reexamine the level of viral specific RNA in CEF of various GS genotypes. We find that GS/GS CEF contain approximately 100 copies of viral RNA per cell, gs/gs CEF contain no detectable viral RNA, and the heterozygote GS/gs CEF contain approximately 50 copies of viral specific RNA per cell. These results suggest that the GS gene controls production of RAV-O RNA sequences in CEF in a "cis" fashion. In RBCs, however, the expression of the RAV-O genome is independent of the GS gene, with both GS/GS and gs/gs RBCs containing roughly equivalent amounts of viral specific RNA. Our results suggest that the chromosomal structure of the endogenous viral genes is independent of the GS gene, and that the GS gene is cis-acting and tissue-specific.