Abstract
Various rat cell lines have been analyzed for expression of endogenous RNA homologous either to RT21C, a typical rat type C virus, or to Kirsten sarcoma virus. Cells have been found that express either (i) high levels of RNA homologous to RT21C rat type C virus and low levels of RNA homologous to Kirsten sarcoma virus (RT21Chigh,sarclow) or (ii) high levels of RNA homologous to Kirsten sarcoma virus and low levels of RNA homologous to typical rat type C virus (sarchigh, RT21Clow). The properties of these two classes of cell lines have been compared. Each type of cell contains an equal amount of the expressed RNA on polysomes. Cell lines that are RT21Chigh produce abundant rat p30 nad p12 structural proteins and release rat type C particles containing viral RNA and reverse transcriptase into supernatant fluids from these cultures. Cell lines that are sarchigh,RTC21Clow have no detectable rat viral p12 protein and no p30 protein immunoreactive in even broad interspecies radioimmunoassays, and do not release type C particles into the supernatant from the cultures. When the particle-negative cell lines are superinfected with heterologous mouse or wooly type C viruses or are producing typical rat type C virus particles, the endogenous sarcoma virus-specific RNA is secreted from these cells. The sarcoma virus-specific RNA can be transcribed in complementary DNA in the endogenous reverse transcriptase reactions carried out in vitro with such virus preparations. However, exposure of cells that are permissive to the helper virus with the particles containing sarcoma virus-specific RNA has not yet resulted in cell transformation or in the synthesis of these RNA sequences. The results suggest: (i) that the first step in the genesis of sarcoma viruses involves the packaging of this expressed sarcoma virus-specific RNA in helper viral particles; (ii) that efficient transmission of the sarcoma virus-specific RNA requires additional events; and (iii) that the formation of a stable sarcoma virus by recombination between the helper viral genome and part of the rescued sarcoma virus-specific RNA is much less common event than the rescue process itself.