Derepression of a novel class of vaccinia virus genes upon DNA replication.

A novel class of vaccinia virus genes, called intermediate, is expressed immediately post-replication and prior to the onset of late gene transcription. Intermediate transcription is dependent on trans-acting factors which are present in an active state in virus-infected cells prior to the onset of DNA replication. Plasmid-borne intermediate genes transfected into vaccinia-virus infected cells are expressed prior to DNA replication, whereas the copies within the viral genome are repressed. DNA replication is essential for activation of viral intermediate transcription and de novo protein synthesis is not required post-replication. In contrast, activation of late transcription depends on DNA replication and continued de novo protein synthesis. Therefore, a subset of intermediate proteins is likely to be trans-activators of late gene transcription. Cell-free extracts differentially transcribe early, intermediate and late genes in a way similar to the temporal expression observed in vivo. A cascade model is discussed for the regulation of gene expression during the viral life-cycle.