Characteristics of a conditional mutant of Rous sarcoma virus defective in ability to transform cells at high temperature

Abstract

A conditional mutant of SR-RSV-D with a highly reduced ability to transform chicken cells at high temperature (41°) was obtained by mutagenization with 5-fluorouracil. This ts mutant (FU-19) retains its capacity to replicate at 41°C whereas it produces 30- to 50-fold fewer foci of Rous cells in monolayer cultures at this temperature than at 37°. Cells grown at 41° transform to Rous cells within a few hours after being shifted to 37°, and cells transformed at 37° revert also within a few hours to a normal morphology and behavior after being shifted to 41°. Hence, maintenance of the transformed state depends on the continuous expression of some specific viral gene(s). When high molecular weight dextran sulfate is added to the medium, the number of residual foci produced at the restrictive temperature, or still observed after shifting to this temperature, is considerably increased. This suggests that the expression of the transforming viral gene(s) alters the cell surface even at 41°—the mutant virus being leaky—and that this alteration produces transformation in a higher number of cells when cellular adhesion is reduced by polyanions. On the other hand, transformation after shifting to 37° requires only protein synthesis, i.e., presumably translation of the transforming information of viral RNA, whereas “detransformation” after shifting to 41° apparently requires no macromolecular synthesis. FU-19 also fails to produce tumors at 41° on the chorioallantoic membrane of embryonated eggs, and sarcoma cells produced in the CAM at 37° disappear after shifting to 41°. Hence, transformation of chicken cells in vivo as well as in vitro is a reversible phenomenon and similarly depends on the continuous expression of the viral transforming gene(s).