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

A new method was developed to localize the site of transcription withi n the nucleolus. Incorporation of digoxigenin-labeled UTP in nucleoli of human PHA-stimulated lymphocytes and in equine melanoma cells was v isualized with antibodies against digoxigenin and secondary gold-label ed 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 contras t, did not display significant labeling above background levels. This finding corroborates the view that the dense fibrillar component is th e site of RNA transcription in the nucleolus.