Control of expression of the vaccinia virus thymidine kinase gene.

mRNA extracted from vaccinia virus-infected cells early after infection directs cell-free synthesis of enzymatically active viral thymidine kinase (Hruby and Ball, Virology, in press). We used this assay for a specific vaccinia virus mRNA to study the induction and repression of the viral thymidine kinase gene during infection of thymidine kinase-deficient L-cells. As observed previously by other workers, the synthesis of thymidine kinase occurred immediately after infection but was switched off after 4 h later. We observed similar kinetics of accumulation and shutoff under conditions where viral DNA synthesis and late gene expression were inhibited. Cell-free translation of mRNA from infected cells showed that the concentration of functional message for viral thymidine kinase reached a peak 3 to 4 h after infection and then decreased with a half-life of about 1 h. These kinetics indicated that significant levels of thymidine kinase mRNA persisted in cells which had stopped synthesizing the enzyme. Under conditions where late gene expression was inhibited, high concentrations of functional mRNA could be isolated from cells at late times after infection. On the basis of these results, we conclude that the repression of thymidine kinase expression is mediated at the translational level by one or more early or delayed early viral genes. Repression is accompanied by, but does not depend on, the inactivation or degradation of thymidine kinase mRNA, which is a late gene function.