Methylation of nucleolar RNA in HeLa cells studied by autoradiography.

Methylation of nucleolar RNA was studied by autoradiography in HeLa cells using L-[methyl-3H]methionine and S-adenosyl-L-[methyl-3H]methionine as radioactive precursors. Pulse-labeling experiments show that nucleolar RNA methylation occurs on the newly synthesized RNA at the nucleolar fibrillar RNP component and mostly on the fibrillar ring of fibrillar centers, where pre-rRNA is being synthesized. Pulse-chase experiments show a shift of silver grains from the nucleolar fibrillar RNP component to the nucleolar granular component first and then to the cytoplasm. Labeling of nucleolar RNA via specific methylation permits the study of intranucleolar processing of pre-rRNA and confirms the sequence of labeling of the two nucleolar RNP components observed with radioactive uridine.