Interferon prevents the generation of spontaneous deletions at the left terminus of vaccinia virus DNA.

In this report we have shown that Friend erythroleukemia cells persistently infected with vaccinia virus maintain the persistent infection even after 1 year of continuous interferon (IFN) treatment. The persistently infected cultures were responsive to IFN as determined by their ability to induce 2-5A synthetase, to increase the intracellular levels of 2-5A, and to cause rRNA cleavage. While large deletions at the left terminus of vaccinia DNA occurred readily in the virus population from untreated cells, IFN completely suppressed the generation of these spontaneous deletions. Removal of IFN from these cultures led to the appearance of similar deletions at the left terminus of the viral genome. The regions deleted contain more than half of the left-end inverted terminal repetition of the vaccinia genome. These findings show that IFN alters specific events associated with the generation of vaccinia DNA deletions.