Proviruses of avian sarcoma virus are terminally redundant, co-extensive with unintegrated linear DNA and integrated at many sites

We have analyzed the DNA from 15 clones of avian sarcoma virus (ASV)-transformed rat cells with restriction endonucleases and molecular hybridization techniques to determine the location and structure of proviral DNA. All twenty units of proviral DNA identified in these 15 clones appear to be inserted at different sites in host DNA. In each of the ten cases that could be sufficiently well mapped, entirely different regions of cellular DNA were involved. Thus ASV DNA can be accommodated at many positions in cellular DNA, but the existence of preferred sites has not been excluded. Six of the 15 clones carry only one normal provirus, two contain two normal proviruses, and seven harbor either one or two proviruses that appear anomalous in physical mapping tests. Both ends of at least 18 proviruses, however, were found to contain sequences specific to both the 3′ and 5′ termini of viral RNA. The organization of these terminally redundant sequences appeared identical to that of the 300 base pair (bp) repeats found at the ends of unintegrated linear DNA (Shank et al., 1978). Proviral DNA is therefore co-extensive, or nearly co-extensive, with unintegrated linear DNA and has a structure we denote as CELL DNA-3′5′—3′5′-CELL DNA. Three of the four anomalous proviruses which were fully analyzed were deletion mutants lacking 25–65% of the genetic content of ASV; the fourth provirus had a novel site for cleavage by Eco RI but was otherwise normal. Tests for the biological competence of proviral DNA, based upon rescue of transforming virus after fusion with chicken cells, were generally consistent with the physical mapping studies.