Transcriptional transactivator that activates the viral internal promoter (IP), thereby enhancing its own expression. This transactivation is repressed by nuclear factor I. Also transactivates the long terminal repeat (LTR) promoter, thereby inducing structural gene expression, initiating the late phase of infection. It is therefore a key regulator of viral gene expression. It directly binds to and activates DNA target sites of viral promoters and those of distinct cellular genes. Required for viral replication (By similarity). Homodimer or homomultimer. Forms complexes with the host nuclear factors NFIA, NFIB, NFIC or NFIX (By similarity). The first 88 residues are shared by isoform Bel-1 and isoform Bet.