Analysis of endogenous avian retrovirus DNA and RNA: Viral and cellular determinants of retrovirus gene expression

Abstract

We have characterized endogenous avian retrovirus DNAs and RNAs in uninfected white leghorn chicken embryos. The embryos we studied include representations of four phenotypic classifications of chickens (gs − chf − , gs + chf + , gs − chf +, and V +), and contained the endogenous proviruses evl, 2, 3, 4, 5, 6, 8, 9, and 15. We confirmed the previously described phenotypes and restriction maps of evl, 3, 4, 5, 6, 8, and 15, and we confirmed the phenotype for ev9 and derived a restriction map for this locus. Evaluation of the structure of the individual proviruses and characterization of the nuclear and cytoplasmic viral RNA isolated from embryos of defined genotype yielded the following major conclusions. (i) Viral RNA could not be detected for ev4, ev5, ev8, or ev15. (ii) The amount of stable RNA produced from evl, ev3, ev6, or ev9 varied over a range of at least 100- to 300-fold and was at least 10- to 1000-fold lower than the amounts found in cells productively infected with exogenous avian retroviruses. (iii) A deletion in the provirus at ev3 can account for three abnormalities: the production of a viral protein that results from fusion of internal regions of the gag and pol genes, the abnormal structure of a subgenomic viral RNA, and failure of the abnormal RNA to appear in the cytoplasm. (iv) A deletion in the provirus at ev6 gives rise to at least two major anomalies: the initiation of transcription at an upstream cellular promoter, and the abnormal metabolism of an ev6 viral RNA. (v) ev9 has an ostensibly normal structure; nevertheless, the metabolism of an RNA arising from this locus is aberrant. We conclude that the expression of endogenous retrovirus genes in chickens is subject to control by both viral and cellular determinants.