Non-activated p53 co-localizes with sites of transcription within both the nucleoplasm and the nucleolus

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Abstract
The p53 tumour suppressor functions as a sensor of genotoxic stress and, once activated, induces cell growth arrest or apoptosis. The precise intranuclear localization of latent p53 protein in non-stressed cells is unknown. Such information is essential in order to understand how relatively few molecules of p53 can detect and respond to DNA damage. Here we present the first detailed supramolecular localization of p53 in the nuclei of cells under normal conditions of growth. We show that soluble, non-bound p53 is released by permeabilization, leaving structurally bound p53 in both the nucleus and nucleolus. In situ biochemical studies reveal (i) that nuclear-bound p53 is tethered by RNA (directly or indirectly) and (ii) that a sub-population of nuclear-bound p53 co-localizes with sites of RNA synthesis. Transcriptional co-localization appeared to be independent of p53 conformation but dependent upon its quaternary structure. In the nucleolus p53 was observed at sites of rRNA synthesis and also adjacent to such sites. In contrast, nucleolar hdm-2 (shown by others to complex p53 and 5S RNA) was excluded from sites of rRNA synthesis. Our discovery that p53 is physically linked with sites of transcription may explain how relatively few p53 protein molecules can monitor genetic stress and respond preferentially to damage of actively transcribed genes.