A temperature-sensitive mutation of the vaccinia virus E11 gene encoding a 15-kDa virion component

The ts49 mutation of vaccinia virus WR was mapped by marker rescue to the E11 gene encoding a 15-kDa polypeptide. During synchronous infection of BSC40 cells with wild-type virus, immunoreactive E11 protein accumulated in parallel with the onset of late protein synthesis. Immunoblotting of extracts of wild-type virions showed that the E11 protein was encapsidated within the virus core. A normal temporal pattern of viral protein synthesis was observed in cells infected with ts49 at the nonpermissive temperature (40 degrees) and normal-appearing ts49 progeny virions were observed by electron microscopy. Sequencing of the E11 gene of ts49 revealed a single amino acid substitution, Gly(66)Arg, in the mutant E11 polypeptide. The steady-state level of E11 protein during ts49 infection was much lower than that observed during infection with wild-type virus. This was the case at both the permissive and nonpermissive temperatures. We discuss two possible explanations for the thermosensitive growth of ts49: (i) that virus infectivity requires a threshold level of active E11 protein or (ii) that E11 function is conditionally essential.