Measurement of endogenous leukosis virus nucleotide sequences in the DNA of normal avian embryos by RNA-DNA hybridization

Abstract

Rous associated virus type O (RAV-O) is a subgroup E avian leukosis virus selected as an example of “endogenous” leukosis viruses which are associated with normal avian embryo cells. DNA sequences complementary to RAV-O 60–70S RNA genome were detected and quantitated in DNA from normal avian embryos by the technique of RNA-DNA hybridization with DNA excess. DNA from “leukosis free” white leghorn chicken embryos, as well as from line 7 chicken embryos, contained sequences related to at least 70%, if not all, of RAV-O RNA sequences. A tentative estimate of gene frequency is low, in the range of two to four copies of RAV-O per cell genome. Some difficulties in precisely calculating gene frequency by this method are discussed. Embryos of ring-necked pheasant contain only about 10% of the RAV-O genome. These findings are not affected by the presence or absence of viral group specific ( gs) antigen in chicken or pheasant embryos. Embryos of Japanese quail, a species which lacks a known leukosis virus and does not synthesize avian gs antigen, contained DNA sequences specifically related to no more than 4% of the RAV-O genome. The data document the specificity and sensitivity of this hybridization technique and provide additional firm support for the proposition that normal avian cells carry endogenous avian leukosis virus in the form of DNA proviruses.