On the structure, genesis and significance of DNA duplications at the Rous sarcoma proviral insertion sites in Rat-1 cells

We have previously shown that most Rous sarcoma proviruses in Rat-1 cells are simple insertions, at apparently random sites, and are not transcribed. A small minority of simple insertions are transcribed and these show a bias to insertion at sites closely 3 ' to presumptive cellular CG-rich islands. However, most transcribed proviruses are complex, typified by conti-guous duplications of proviral DNA 5 ' to a complete proviral unit. The cellular sites of these complex insertions are unknown and their structure and signifi-cance incompletely documented. We report here more extensive analyses of proviral duplications, with the following findings. The 5 ' duplications predominantly involve proviral regions known to contain enhancer functions, substantiating earlier data. The cellula