Mutant of Murine Sarcoma Virus

Morphological transformation of normal rat kidney cells by a murine sarcoma virus was found to be cold-sensitive. Cells transformed by the virus expressed their transformed phenotype at the permissive temperature (390) but not at the nonpermissive temperature (330), as judged by the criteria of morphological changes and colony-forming ability on monolayers of normal rat kidney cells. Cold-sensitive expression of transformation was specific for focus-derived normal rat kidney cells transformed by the virus, readily reversible, and not lost during serial propagation of the cells. The genome of the murine sarcoma virus can be rescued by superinfection with Moloney leukemia virus at the permissive or nonpermissive temperature, and the rescued virus exhibited the same cold-sensitive properties as the original transforming virus. These results suggest that maintenance of the transformed state is continuously dependent on a cold-sensitive viral function. Clonal isolation in normal rat kidney (NRK) cells of murine sarcoma virus (MSV) from the MSV-Moloney leukemia virus (MSV-MoLV) complex has been described (1). Two types of MSV-transformed NRK cells were obtained using an agar overlay cloning method: (a) NRK(MSV-1)-transformed cells produced a virus, MSV-1 (NRK), that could transform NRK cells but failed to produce progeny virus in the newly infected cells; and (b) NRK(MSV-6)-transformed cells produced a virus, (MSV-7(NRK), that failed to transform or replicate in any cell tested. MSV-1(NRK) contains genetic information for transformation, but has lost replication functions. These replication functions can be supplied by MoLV (1). MSV-6(NRK) has lost both transformation and replication functions and is deficient in DNA polymerase activity (2). MSV-1 (NRK) provides a useful tool for examination of events leading to transformation of NRK cells, since the virus is blocked in replication functions. The data to be presented demonstrate that maintenance of transformation of NRK cells induced by MSV-1 (NRK) is cold-sensitive. MATERIALS AND METHODS Cell Cultures and Viruses. Uninfected NRK cells and MSVtransformed or MoLV-infected NRK cells were grown in Auto Pow minimal essential medium (Flow Laboratories, Rockville, Md.) supplemented with 10% fetal-bovine serum as described (1). Viral pools were made from supernatant fluids of chronically infected cell cultures. Viral Infection and Assay. Infection procedures using DEAE-dextran to enhance infectivity have been described (1, 3). MSV was assayed by focus formation in NRK cells (1), and MoLV was assayed by the XC cell plaque method (4).