Reverse genetics analysis of poxvirus intermediate transcription factors.

Vaccinia virus transcription is regulated in three stages. An intermediate transcription factor, comprised of virus-encoded polypeptides A8 and A23, was previously identified by in vitro analyses. To investigate its role, we engineered cells that stably expressed both subunits and complemented the replication of A8 and A23 deletion mutant viruses. Without A8 or A23, viral early gene expression and DNA replication occurred but intermediate and late gene expression and resolution of genome concatemers were not detected.