Synthesis and integration of avian sarcoma virus DNA

In this paper, we have presented data regarding the integration of viral DNA and the persistence of unintegrated viral DNA. We know that integration occurs with specificity with respect to the viral genome, and yet it remains to be established which, if any, of the unintegrated DS DNA species-either the linear or the covalently closed circular forms-is the immediate precursor to the DNA that does become integrated. The results of the present studies indicate that the viral DNA species that are able to persist in an unintegrated state may be transcribed to yield new virions. Furthermore, these species may not be intermediates as far as the integration pathway, but may instead be terminal products of a synthesis reaction that has proceeded beyond the point where integration normally occurs. Varmus et al. (1977) have shown that integration occurs only into newly replicated cellular DNA. Therefore, we would speculate that integration may occur by an interaction between a replication fork of the host cell DNA and an earlier, non-double-strand form of viral DNA.