THE UPTAKE OF URIDINE IN THE NUCLEOLUS OCCURS IN THE DENSE FIBRILLAR COMPONENT - IMMUNOGOLD LOCALIZATION OF INCORPORATED DIGOXIGENIN-UTP AT THE ELECTRON-MICROSCOPIC LEVEL

A new method was developed to localize the site of transcription within the nucleolus. Incorporation of digoxigenin-labeled UTP in nucleoli of human PHA-stimulated lymphocytes and in equine melanoma cells was visualized with antibodies against digoxigenin and secondary gold-labeled antibodies at the electron-microscopic level. This approach offers much higher spatial resolution than the autoradiographic methods used so far. In both types of cells digoxigenin-UTP was incorporated mainly into the dense fibrillar component; the fibrillar centers, in contrast, did not display significant labeling above background levels. This finding corroborates the view that the dense fibrillar component is the site of RNA transcription in the nucleolus.