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Visualization of focal sites of transcription within human nuclei.

HeLa cells were encapsulated in agarose microbeads, permeabilized and incubated with Br‐UTP in a ‘physiological’ buffer; then sites of RNA synthesis were immunolabelled using an antibody that reacts with Br‐RNA. After extending nascent RNA chains by < 400 nucleotides in vitro, approximately 300–500 focal synthetic sites can be seen in each nucleus by fluorescence microscopy. Most foci also contain a component of the splicing apparatus detected by an anti‐Sm antibody. alpha‐amanitin, an inhibitor of RNA polymerase II, prevents incorporation into these foci; then, using a slightly higher salt concentration, approximately 25 nucleolar foci became clearly visible. Both nucleolar and extra‐nucleolar foci remain after nucleolytic removal of approximately 90% chromatin. An underlying structure probably organizes groups of transcription units into ‘factories’ where transcripts are both synthesized and processed.