Vaccinia virus gene D6R encodes the small subunit of the virion early gene transcription initiation factor. Three temperature-sensitive mutations have been mapped to this gene. The biochemical phenotype exhibited by each mutation was examined. All mutants displayed altered viral protein synthesis in pulse-labelling analyses at both the permissive and non-permissive temperatures. The onset of early protein synthesis was delayed, and the rate of early protein synthesis was reduced in each case. Furthermore the shut-off of both host and early protein synthesis was delayed. In pulse-chase experiments, the stability of the D6R protein in E93- or C46-infected cells was shown to be reduced at 40 °C relative to that at 31 °C. Early mRNA was quantified in cells at 2 h post-infection and shown to be reduced substantially. The ability of each mutant virus to support transcription was examined at both temperatures and, of the three mutants, only S4 transcription was shown to exhibit reversible temperature sensitivity.


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