Temperature-sensitive mutant in the vaccinia virus E6 protein produce virions that are transcriptionally inactive

The vaccinia virus E6R gene encodes a late protein that is packaged into virion cores. A temperature-sensitive mutant was used to study the role of this protein in viral replicative cycle. Cts52 has a P226L missense mutation in the E6R gene, shows a two-log reduction in plaque formation, but displays normal patterns of gene expression, late protein processing and DNA replication during infection. Mutant virions produced at 40 °C were similar in their morphology to wt virions grown at 40 °C. The particle to infectivity ratio was 50 times higher in purified Cts52 grown at 40 °C when compared to the mutant grown at permissive temperature. In vitro characterization of Cts-52 particles grown at 40 °C revealed no differences in protein composition or in DNA content and the mutant virions could bind and enter cells. However, core particles prepared from Cts52 grown at 40 °C failed to transcribe in vitro. Our results show that E6 in the virion has either a direct or an indirect role in viral transcription.