Different mechanisms for morphologic reversion of a clonal population of murine sarcoma virus-transformed nonproducer cells.

Abstract Morphologic revertants of a clonal line of BALB/3T3 nonproductively transformed by Kirsten murine sarcoma virus were isolated by selection in methylcellulose semisolid suspension medium. Each revertant clone was obtained from a separate mutagenized cell culture in an attempt to isolate the widest spectrum of genetically altered cells. The newly isolated revertants were indistinguishable from BALB/3T3 in morphology and in vitro growth properties. One group, like a previously reported series (Greenberger, J. S., Anderson, G. R., and Aaronson, S. A. (1974). Cell 2 , 279–286.) was shown to contain reversibly altered sarcoma viral genomes. These cell lines expressed high levels of sarcoma viral RNA, and wild-type transforming virus became rescuable at a time when cells in the population spontaneously retransformed. This occurred at low frequencies ranging from one in 10 6 to 10 8 cell generations. There was no evidence of complementation or recombination between the altered sarcoma viruses present in these revertant lines. A second group of revertants was very stable to retransformation and failed to demonstrate rescuable sarcoma virus. These cell lines contained no more sarcoma viral RNA or DNA than uninfected BALB/3T3 indicating that these revertants arose from loss of the sarcoma virus genome. Thus, the present studies indicate that both alteration and loss of sarcoma viral information are reproducible causes of reversion to the nontransformed state in this system.