The mechanism employed by DNA tumor viruses to inhibit p53-dependent transcription from chromatin is poorly understood. Here, we use in vitro-reconstituted chromatin and UV-irradiated cells to define the mechanism of human papillomavirus E6 oncoprotein in repressing p53-dependent transcription. We demonstrate that E6 does not prevent p53 or p300 recruitment to the chromatin but inhibits p300-mediated acetylation on p53 and nucleosomal core histones. This suppression of protein acetylation requires the E6-interacting regions of p300. Moreover, E6 mutants unable to interact with p53 or p300, but not deficient in inducing p53 degradation, fail to inhibit p53-mediated activation, indicating that a p53-E6-p300-containing protein complex is critical for repressing p53-targeted gene activation. That E6 acts as a molecular switch converting p53-p300 from an activating complex to a repressing entity on the chromatin, which occurs independently of E6AP-mediated protein degradation pathway, may represent a general mechanism for gene regulation.
