Genetic Variation in the RNA Transcripts of Endogenous Virus Genes in Uninfected Chicken Cells

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Abstract
Uninfected cells from two different phenotypes of chicken embryos express significant amounts of endogenous viral information, though they do not produce virus particles. Cells of the phenotype gs + chf + are positive for both group-specific (gs) antigens and chicken helper factor ( chf ) activity, whereas cells of a second phenotype, gs L chf + (h E ), demonstrate noncoordinate expression of these two viral activities (very low amounts of gs antigens, but extremely high helper activity). RNA from these cells was analyzed to determine the size, genetic content, and relative abundance of virus-specific RNAs in cells of each phenotype. Two major size classes of polyadenylic acid-containing RNA, homologous to the avian leukosis virus genome, were detectable in cells of both types. The larger RNA, which contained most of the sequences of the leukosis virus genome, was of different sizes in the two phenotypes, 31S in gs + chf + cells but 35S in the noncoordinate cell type. Analysis of the viral RNA with gene-specific complementary DNA probes revealed the following characteristics. (i) The 31S RNA appeared to lack portions of the gag and pol genes. (ii) A smaller RNA species, which sedimented at 21S in both cell types, was a transcript of the 3′-proximal portion of the viral genome, consisting of the env gene and the “common” sequences. (iii) The amount of env -specific RNA in the 21S region was more than six times higher in the noncoordinate cell type than in the gs + chf + cells; this difference was concordant with the 5- to 10-fold higher chf activity in the noncoordinate cells. (iv) The endogenous viral RNA in uninfected cells and the RNA from Rous-associated virus-0 virions hybridized only partially with DNA complementary to the common region of the Rous-associated virus-2 genome, whereas the RNA of all exogenous viruses tested hybridized almost completely to this complementary DNA. Small amounts of src -specific polyadenylated RNA were also present in uninfected chicken cells. This RNA sedimented as a single peak at 26S and was not covalently linked to any other identifiable virus-specific RNA sequences. The amount of src RNA was the same in the above two types of expression-positive cells and also in cells that were gs − chf − , indicating that the transcription of the cellular sequences homologous to the src gene is independent of the transcription of the other endogenous viral genes.