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The immediate-early protein, bICP0, of Bovine herpesvirus 1 (BHV-1) transactivates viral promoters and stimulates productive infection. bICP0 is expressed constitutively during productive infection, as its gene contains an immediate- early and an early promoter. Like other ICP0 homologues encoded by members of the subfamily Alphaherpesvirinae, bICP0 contains a zinc RING finger located near its N terminus. Mutations that disrupt the bICP0 zinc RING finger impair its ability to activate transcription, stimulate productive infection, inhibit interferon-dependent transcription in certain cell types and regulate subnuclear localization. bICP0 also interacts with a cellular chromatin-remodeling enzyme, histone deacetylase 1 (HDAC1), and can relieve HDAC1-mediated transcriptional repression, suggesting that bICP0 inhibits silencing of the viral genome. In this study, it was shown that bICP0 interacted with the histone acetyltransferase p300 during productive infection and in transiently transfected cells. In addition, p300 enhanced BHV-1 productive infection and transactivated a late viral promoter (gC). In contrast, a CH3-domain deletion mutant of p300, which is a dominant-negative mutant, did not activate the gC promoter. bICP0 and p300 cooperated to activate the gC promoter, suggesting that there is a synergistic effect on promoter activation. As p300 can activate certain antiviral signaling pathways (for example, interferon), it was hypothesized that interactions between p300 and bICP0 may dampen the antiviral response following infection.