Promyelocytic leukemia (PML) nuclear bodies are protein structures that donot accumulate RNA

The promyelocytic leukemia (PML) nuclear body (also referred to as ND10, PO D, and Kr body) is involved in oncogenesis and viral infection. This subnuc lear domain has been reported to be rich in RNA and a sits of nascent RNA s ynthesis, implicating its direct involvement in the regulation of gene expr ession. We used an analytical transmission electron microscopic method ro d etermine the structure and composition of PML nuclear bodies and the surrou nding nucleoplasm. Electron spectroscopic imaging (ESI) demonstrates that t he core of the PML nuclear body is a dense, protein-based structure, 250 nm in diameter, which does not contain detectable nucleic acid. Although PML nuclear bodies contain neither chromatin nor nascent RNA, newly synthesized RNA is associated with the periphery of the PML nuclear body, and is found within the chromatin-depleted region of the nucleoplasm immediately surrou nding the core of the PML nuclear body. We further show that the RNA does n ot accumulate in the protein core of the structure. Our results dismiss the hypothesis that the PML nuclear body is a site of transcription, but suppo rt the model in which the PML nuclear body may contribute to the formation of a favorable nuclear environment for the expression of specific genes.