Small fragments of herpesvirus DNA with transforming activity contain insertion sequence-like structures

A 737-base-pair fragment of herpes simplex virus type 2 DNA with morphological-transforming ability was identified by transfecting into rodent cells deleted fragments of the left-hand end of the Bgl II N fragment region (map position 0.58-0.625), which were constructed in vitro. The transforming sequences lie within the coding region for a Mr 61,000 protein, but the fragment itself does not appear to specify a viral polypeptide. Contained within the transforming fragment are sequences that can be drawn as a stem-loop structure flanked by direct repeats, similar to an insertion sequence-like element. An insertion sequence-like structure was also found in a small fragment of human cytomegalovirus DNA that has transforming activity. Possible mechanisms of herpesvirus transformation are discussed.