ev 3, a structural gene locus for endogenous virus, segregates with the gs+chf+ phenotype in matings of line 63 chickens

Individual ev loci determine characteristic patterns of endogenous retroviral envelope antigen expression during lymphocyte maturation

A comparative study of the expression of retroviral envelope antigen by T and B lymphocytes at different maturational stages was made with chickens typed for genomic ev loci. All lymphocyte subclasses from chf(-) chickens were negative for expression of retroviral envelope antigen. By contrast, cells of the lymphocyte lineage from all the chf(+) lines examined were positive for expression, with the levels of envelope antigen in the lymphoblasts exceeding the levels in the immature and resting cells. This pattern of lymphocyte expression was found in lines of chickens in which the expression of chf in fibroblasts was encoded by the ev 3, ev 6, or ev 9 loci. Plasma cells from those chf(+) lines possessing ev 6 expressed higher levels of envelope antigen than the lymphoblasts, an effect not observed with the chf(+) lines lacking ev 6.

The influence of the ev 3 locus on the inducibility of serum antibody reactivity for envelope glycoprotein group-specific determinants

Chickens segregating for the ev 3 locus were bred by backcross matings of line 6(3) to line 15B. Analysis of RAV-1-infected segregants indicated that inducibility of antibody reactivity for envelope glycoprotein group-specific determinants correlated with the absence of ev 3, whereas noninducibility correlated with the presence of ev 3. Since the ev 3(+) and ev 3(-) segregants possessed similar genetic backgrounds, these results provide direct evidence that the ev 3 locus determines the phenotype of noninducibility.

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.

Chromosomal clustering of five defined endogenous retrovirus loci in White Leghorn chickens

By using in situ hybridization, we localized three endogenous chicken retrovirus loci associated with the gs- chf- phenotype to distinct regions on chromosome 1. These three loci, ev4, ev5, and ev8, along with the two other known gs- chf--associated loci in White Leghorn chickens, ev1 and ev13, appear to be evenly distributed along chromosome 1, occurring approximately every 1.8 X 10(7) base pairs. The presence of all five loci on chromosome 1 represents a highly nonrandom distribution and all but eliminates a mechanism of independent random integration events for the generation of these loci.

5'-terminal deletions are a common feature of endogenous retrovirus loci located on chromosome 1 of White Leghorn chickens

My previous studies demonstrated that chromosome 1 has all five of the endogenous retrovirus loci associated with nonexpression of viral proteins (gs- chf-) in White Leghorn chickens. The current study was done to localize the two defective endogenous retrovirus loci, ev3 and ev6, to determine whether nonexpression of the viral loci on chromosome 1 is a transcriptional prerequisite or a result of an underlying structural defect. The structure of ev6 is every similar to that of two other gs- chf- -associated loci, ev4 and ev5; all three contain a 5'-terminal deletion that eliminates the viral transcriptional promoter. Unlike the gs- chf- -associated loci, ev6 apparently uses an adjacent cell promoter to produce high levels of viral glycoprotein. The ev6 locus was located on the long arm of chromosome 1 by in situ hybridization, thus indicating that transcription of an endogenous retrovirus locus on chromosome 1 can occur. However, the 5'-terminal deletion in several viral loci on chromosome 1, but not on other chromosomes, suggests that the deletion may result from the events that generate or maintain these loci in the White Leghorn population. The additional finding of ev3, which has an internal deletion, on a microchromosome suggests that the mechanism that produced deletions in many chromosome 1 loci is specific for the 5' terminus.

Chromosomal localization of three endogenous retrovirus loci associated with virus production in White Leghorn chickens

Proposed mechanisms for the generation of endogenous retrovirus loci have been examined by determining the chromosomal distribution of these loci by means of in situ hybridization. Unlike the clustering on chromosome 1 of five endogenous retrovirus loci associated with the gs- chf- phenotype A. Tereba and S. M. Astrin, submitted for publication), three loci associated with endogenous retrovirus production (V+ phenotype) were located on three separate chromosomes. ev2, which codes for the prototype endogenous RAV-0 genome in line 7(2) chickens, was localized near the middle of the long arm of chromosome 2, ev7, coding for a noninfectious, inducible genome present in line 15B chickens, was located near the end of the long arm of the Z chromosome. A third V+ locus, designated ev14, was detected near the middle of chromosome 3. This arrangement of V+ loci is consistent with an integration mechanism employing randomly distributed integration sites in the chicken genome. In addition, these data provide evidence suggesting that the gs- chf- -associated loci may have been generated by a different mechanism.

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.

Chromosomal localization of ev-1, a frequently occurring endogenous retrovirus locus in white Leghorn chickens, by in situ hybridization

The distribution of 12 endogenous retrovirus loci from 13 White Leghorn flocks of diverse geographical origin was examined, using a total of 508 birds. Only one locus, designated ev-1, was observed to be almost universally present (506 of 508 birds) and thus predominantly homozygous in this breed of chicken. By contrast, 11 other viral loci were observed at much lower frequencies, 3 to 30%, and were occasionally restricted to individual flocks. To further characterize the commonly occurring ev-1 viral locus and to provide a foundation for determining the mechanism by which these 12 endogenous retrovirus loci were generated, a new technique of indirect in situ hybridization was used to localize the chromosomal position of ev-1. By using a combination of specific probes, the position of ev-1 was localized to a small region on the long arm of chromosome 1. This research represents oen of the first instances in which a defined segment of nonreiterated DNA coding for a known set of genes has been localized to a specific region on a vertebrate chromosome by in situ hybridization.