Mediates, together with protein p6, the early to late transcriptional switch by stabilizing the binding of host RNA polymerase (RNAP) to the late A3 promoter. Activates transcription from the late A3 promoter and represses the main early promoters A2b and A2c by modifying the topology of the sequences encompassing early promoters A2c and A2b and late promoter A3 in a hairpin. Binds to a region of the A3 promoter located between nucleotides -50 and -100 relative to the transcription start site, that presents a sequence-directed curvature. Full induction of this curvature is needed for the transcription activation process. Interacts with host RNA polymerase (via C-terminus). Interacts with DNA; binds to the A2b, A2c and A3 promoters. Belongs to the phi29likevirus late genes activator p4 family.