The role of vaccinia termination factor and cis-acting elements in vaccinia virus early gene transcription termination

Vaccinia virus early gene transcription termination requires the virion form of the viral RNA polymerase (vRNAP), Nucleoside Triphosphate Phosphohydrolase I (NPHI), ATP, the vaccinia termination factor (VTF), and a U₅NU termination signal in the nascent transcript. VTF, also the viral mRNA capping enzyme, binds U₅NU, and NPHI hydrolyzes ATP to release the transcript. NPHI can release transcripts independent of VTF and U₅NU if vRNAP is not actively elongating. However, VTF and U₅NU are required for transcript release from an elongating vRNAP, suggesting that the function of VTF and U₅NU may be to stall the polymerase. Here we demonstrate that VTF inhibits transcription elongation by enhancing vRNAP pausing. Hence VTF provides the connection between the termination signal in the RNA transcript and viral RNA polymerase to initiate transcription termination. We also provide evidence that a second cis-acting element downstream of U₅NU influences the location and efficiency of early gene transcription termination.