Visualization of nuclear architecture is key to the understanding of the association between RNA synthesis and processing. This architecture is obscured by the high density of components in most nuclei. We have developed a method of spreading nuclei and nucleoli that reduces overlap of weakly associated components. Strong interactions among nuclear components are not disrupted by this method. Spread nucleoli remained structurally distinct and functionally competent in ribosomal RNA synthesis. Nascent ribosomal RNA colocalized with RNA polymerase I and fibrillarin, a protein required for processing of ribosomal RNA. Colocalization of nascent transcripts and fibrillarin was seen in nucleoli spread over several microns, suggesting a strong interaction. These data suggest that nucleoli are superassemblies of bipartite domains, each composed of a ribosomal RNA synthesis center tightly associated with areas likely to be involved in ribosomal RNA processing.
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