| AP-1 complexes are a family of transcription factors ubiquitously expressed in different cell types. Transcriptional regulation mediated by AP-1 has been extensively studied; AP-1 mainly works through its cognate sequences, which are found in the regulatory elements in a variety of genes. Current genome-wide promoter occupancy studies indicate that the regulatory specificity is conferred mainly through combinatorial transcription factor binding elements containing non-consensus low affinity sites. However, the question of whether the binding sequences possess intrinsic selectivity for a given family transcription factors, which exhibit overlapping binding preference, is hard to define in living cells. Our in vitro biochemical studies unambiguously demonstrated that distinct AP-1 sites, derived from the HPV-11 upstream regulatory region (URR), indeed exhibit differential binding properties toward distinct recombinant human AP-1 complexes. Importantly, we further disclosed a conserved consensus-like binding site existing near the E6 core promoter across different genital HPVs based on an in-depth analysis of our in vitro experimental data.; It is noteworthy that the investigation of AP-1-mediated regulation of HPV chromatin transcription is significantly facilitated by the strategy of in vivo reconstitution of coexpressed human AP-1 subunits using a polycistronic bacterial expression system to generate various full-length recombinant AP-1 complexes, which were applied to various in vitro functional assays performed in the present work. Previously we found that both AP-1 and p300 are required for activation of HPV chromatin transcription; however, the underlying mechanism remains unclear. We demonstrated that AP-1 recruits p300 to stimulate in vitro HPV chromatin transcription mainly depends on acetylation of nucleosomal histones by p300. Interestingly, p300 can also mediate acetylation of all the subunits of AP-1 complexes, whose DNA binding activity is stimulated by acetylation. Interestingly, we addressed the molecular mechanism and the functional role of p300-mediated acetylation on AP-1 transcription factors. Furthermore, the finding of targeted histone acetylation surrounding the redundantly occupied HPV E6 promoter-proximal AP-1 site demonstrated in living cells implicates a biologically important role played by AP-1 and possibly uncovers a common mechanism underlying the HPV pathogenesis. |
