DE-NOVO SYNTHESIS OF THE EARLY TRANSCRIPTION FACTOR 70-KILODALTON SUBUNIT IS REQUIRED FOR MORPHOGENESIS OF VACCINIA VIRIONS

Vaccinia virus early transcription factor (VETF) is a heterodimeric pr otein that is packaged in virus particles for expression of early gene s during the next round of infection. To investigate additional roles of VETF, we constructed a conditionally lethal recombinant vaccinia vi rus in which the D6R gene, encoding the 70-kDa subunit of VETF, is und er stringent Escherichia coli lac operator control. When cells were in fected with the recombinant virus in the absence of an inducer, synthe sis of the 70-kDa protein was undetectable and the yield of infectious virus was severely reduced. Under these nonpermissive conditions, DNA replication and synthesis of viral proteins other than the one encode d by D6R occurred, suggesting that de novo synthesis of VETF is not re quired for expression of early or late genes during the virus growth c ycle. Electron microscopy, however, revealed that immature virus parti cles and masses of electron-dense material accumulated in the absence of an inducer. We concluded that VETF has a direct role in virion morp hogenesis or is required for expression of a novel subset of genes tha t have such a role.