Activates transcription at late promoters when the sliding clamp is present (PubMed:33602900). Binds to both the host RNA polymerase (RNAP) and the upstream dsDNA (PubMed:33602900, PubMed:8106327). Interacts with the beta subunit of host RNAP RpoB (via flap domain) (PubMed:33602900, PubMed:22135460). Part of the transcription activation complex containing host RNAP, the viral RNA polymerase sigma-like factor, the coactivator gp33, and the sliding clamp (PubMed:33602900). The C-terminus is involved in transcriptional enhancement. Belongs to the Tevenvirinae late transcription coactivator family.