ACTIVE RNA-POLYMERASES ARE LOCALIZED WITHIN DISCRETE TRANSCRIPTION FACTORIES IN HUMAN NUCLEI

Nascent transcripts in permeabilized HeLa cells were elongated by simi lar to 30-2,000 nucleotides in Br-UTP or biotin-14-CTP, before incorpo ration sites were immunolabelled either pre- or post-embedding, and vi sualized by light or electron microscopy. Analogues were concentrated in similar to 2,100 (range 2,000-2,700) discrete sites attached to a n ucleoskeleton and surrounded by chromatin. A typical site contained a cluster (diameter 71 nm) of at least 4, and probably about 20, engaged polymerases, plus associated transcripts that partially overlapped a zone of RNA polymerase II, ribonucleoproteins, and proteins rich in th iols and acidic groups. As each site probably contains many transcript ion units, these results suggest that active polymerases are confined to these sites, which we call transcription 'factories'. Results are c onsistent with transcription occurring as templates slide past attache d polymerases, as nascent RNA is extruded into the factories.