Vaccinia Virus Late Transcription Is Activated in Vitro by Cellular Heterogeneous Nuclear Ribonucleoproteins

Vaccinia virus gene expression is temporally regulated, and three gene classes have been identified: early, intermediate, and late. Several virus-encoded proteins and an activity designated VLTF-X are required for maximum transcription in vitro of a template containing a late promoter. VLTF-X is present in both cytoplasmic and nuclear extracts prepared from uninfected mammalian cells and co-purifies with a late promoter DNA-binding activity. Here, extensive purification of VLTF-X has revealed that heterogeneous nuclear ribonucleoproteins A2/B1 and RBM3 co-purified with in vitro late transcription stimulation. Overexpression and purification of these proteins from Escherichia coli demonstrated that they both complemented for VLTF-X activity in in vitro transcription reactions. These studies identify two host cell factors potentially contributing to poxvirus replication in vivo.