Reversion of murine sarcoma virus transformed mouse cells: variants without a rescuable sarcoma virus

Murine sarcoma virus transformed mouse 3T3 cells, which are negative for murine leukemia virus and which yield sarcoma virus after superinfection with murine leukenmia virus, spotaneously give rise to flat variants front which murine sarcoma virus can no longer be rescued. The revertants support leukemia viruis growth and show an enhanced sensitivity to murine sarcoma superinfection and, like normal cells, do not release RNA-dependent DNA polymerase activity. Because revertants could be obtained with high frequency from progeny of single transformed cells, each cell that containts the sarconma virus genome seems to have the capacity to suppress or eliminate an RNA tumor virus native to its species of origin.

Replication of radiation-induced murine leukemia virus in normal and transformed mouse cells

Autonomous radiation-induced leukemia virus (RadLV) replication could be detected in mouse 3T3 cells by the development of interference with murine sarcoma virus (MSV), the appearance of covert helper activity for defective MSV, and by the induction of cytopathic effect type foci in MSV-transformed, leukemia virus-negative (S+L-) cells. A chronic infection of either 3T3 or S+L- cells with RadLV could be established. Both RadLV infectivity and helper activity were demonstrated in the same peak at a buoyant density of 1.16 g/cm(3). Additionally a soluble inhibitor of MSV focus formation was found which could be separated from infectious RadLV. Examination of cell clones derived from chronically infected 3T3 cells showed that essentially every cell was infected and produced both infectious RadLV and low levels of inhibitor. Quantitative comparisons of autonomously replicating RadLV in normal 3T3 and S+L- cells suggested that RadLV may consist of several populations of virus of varying replicative potential. Apparently 99% of RadLV can be assayed only as helper units in normal cells or as replicative units in S+L- cells. To explain the atypical results, a model for RadLV deficiency is proposed.

Rescue of murine sarcoma virus from a sarcoma-positive leukemia-negative cell line: requirement for replicating leukemia virus

The nature of murine sarcoma virus (MSV) "defectiveness" was investigated by employing an MSV-transformed mouse 3T3 cell line which releases noninfectious virus-like particles. Rescue kinetics of MSV, observed after murine leukemia virus (MuLV) superinfection of these "sarcoma-positive leukemia-negative (S + L -)" mouse 3T3 cells, consisted of a 9- to 12-hr eclipse period followed by simultaneous release of both MSV and MuLV with no evidence for release of infectious MSV prior to the production of progeny MuLV. Addition of thymidine to the growth medium of MuLV-superinfected S + L - cells at a concentration suppressing deoxyribonucleic acid synthesis inhibited the replication of MuLV and the rescue of MSV. MSV production closely paralleled MuLV replication under a variety of experimental conditions. These results suggest that replication of MuLV is required for the rescue of infectious MSV from S + L - cells and that one (or more) factor, produced late in the MuLV replicative cycle, is utilized by both viruses during virion assembly. During the course of these experiments, virus stocks were recovered which contained infectious MSV in apparent excess over MuLV. These stocks were used for generating new S + L - cell lines by simple end point dilution procedures.

Transformation of mouse 3T3 cells by murine sarcoma virus: release of virus-like particles in the absence of replicating murine leukemia helper virus

Small numbers of virus-like particles were observed by electron microscopy in each of two cloned lines of 3T3 cells transformed by murine sarcoma virus, even though these lines were free of detectable quantities of infectious leukemia and sarcoma virus. The morphology and occurrence of the particles were identical to those of the murine leukemia-sarcoma group. Moreover, the particles incorporated uridine and had a buoyant density of 1.16 g/ml in sucrose gradients. No evidence of sarcoma or leukemia virus infectivity was associated with the particles in cells of several susceptible species under various conditions, including both cosedimentation with leukemia virus and infection in the presence of inactivated Sendai virus. The particles may represent a form of murine sarcoma virus deficient in one or more of the viral components necessary for infectivity.