State of the viral DNA in rat cells transformed by polyma virus. II. Identification of the cells containing nonintegrated viral DNA and the effect of viral mutations

Abstract

F2408 rat [fibroblast] cells transformed by polyoma virus contain integrated and nonintegrated viral DNA. The presence of nonintegrated viral DNA is under the control of the A early viral function. Polyoma ts-a-transformed rat cells lose the free viral DNA when grown at the nonpermissive temperature (40.degree. C), but they reexpress it 1-3 days after they are shifted back to the permissive temperature. Rat cells transformed by a late viral mutant, ts-8, contain free viral DNA at permissive and nonpermissive temperatures. Treatment of the transformed rat cells with mitomycin C produces a large increase in the quantity of free viral DNA and some production of infectious virus. Experiments of in situ hybridization with 3H-labeled polyoma complementary RNA as a probe show that only a minority (.apprx. 0.1%) of the transformed cells contain nonintegrated viral DNA at any given time. Possibly, the presence of free viral DNA in polyoma-transformed rat cells is caused by a spontaneous induction of viral DNA replication, occurring with low but constant probability in the transformed cell population, and the free viral DNA molecules originate from the integrated ones, probably through a phenomenon of excision and limited replication.