ISOLATION AND PRELIMINARY CHARACTERIZATION OF TEMPERATURE-SENSITIVE TRANSFORMATION MUTANTS OF MOLONEY SARCOMA-VIRUS

Temperature-sensitive (ts) transformation mutants of Moloney murine sarcoma virus were isolated from uv-irradiated viral stocks using a selection and screening procedure based on the ability of MSV-transformed NRK cells to grow in methyl cellulose or agar suspension. Mutants isolated by this procedure formed colonies in agar 1000- to 10,000-fold more efficiently at 34° than at 39°. They also exhibited a transformed morphology and elevated hexose transport levels at 34°, but were phenotypically normal at 39°. Both morphology and hexose transport showed transformed → normal and normal → transformed conversion within 12-48 hr of a temperature shift from 34 to 39° and 39 to 34° respectively. In contrast, ts-transformed cells suspended in agar and incubated at 39° for 24 hr showed a 90% reduction in colony-forming ability when the plates were returned to 34°. Superinfection of nonproducer ts-transformed cells with leukemia virus resulted in the rescue of ts MSV. Rescued supernatants also contained a high proportion (10%) of wt MSV. Repeated cloning of ts mutants, either as virus or cells, did not significantly affect the proportion of ts or wt virus rescued. The ability of ts-transformed cells to express the transformed phenotype at 39° could be restored by wt MSV superinfection, but not by MLV superinfection. © 1979.