Type C particle-positive and type C particle-negative rat cell lines: characterization of the coding capacity of endogenous sarcoma virus-specific RNA

Abstract

Various rat cell lines [normal kidney NRK, uterine carcinoma HTP, testicular carcinoma fibroblast WR123, plasmacytoma fibroblast IR162, embryo fibroblast FRE clone 2, thymus RT21C, leukemia W/Fu] were analyzed for expression of endogenous RNA homologous to RT21C, a typical rat type C virus, or to Kirsten sarcoma virus. Cells were found that express high levels of RNA homologous to RT21C rat type C virus and low levels of RNA homologous to Kirsten sarcoma virus (RT21Chigh;sarclow), or express 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 2 classes of cell lines were compared. Each type of cell contains an equal amount of the expressed RNA on polysomes. Cell lines that are RT21Chigh produce abundant rat p30 and 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,RT21Clow 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 woolly 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 endogenous reverse transcriptase reactions carried out in vitro with such virus preparations. 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: that the 1st step in the genesis of sarcoma viruses involves the packaging of this expressed sarcoma virus-specific RNA in helper viral particles; that efficient transmission of the sarcoma virus-specific RNA requires additional events; and 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 a much less common event than the rescue process itself.