Proc Natl Acad Sci U S A 1975 Feb;72(2):673-7
The transforming ability in 10% serum medium of the temperature-sensitive mutants of simian virus 40 in the complementation group III (ts640 type mutants) was greatly reduced when the infected rat 3Y1 cells were incubated at the restrictive temperature of 40 degrees or incubated first at 40 degrees for 3 days and then shifted to the permissive temperature of 33 degrees. Transformation did occur efficiently after incubation at 33 degrees or after an initial incubation at 33 degrees for 5 days followed by a shift to 40 degrees. When growth properties of 3Y1 cells transformed at 33 degrees by the group III mutants were examined at 40 degrees, several aspects of the transformed state were rendered temperature-sensitive. These aspects were the ability of cells to grow in low serum (1.5%) medium and to make colonies, in 10% serum medium, on monolayers of untransformed 3Y1 cells and in soft agar. It is concluded that a simian virus 40 gene (cistron III) controls the initiation, as well as at least some aspects of the maintenance, of transformation and that the initiation reaction is a more heat-labile event than the maintenance reaction(s) under the experimental conditions.