The effect of adenosine analogue (DRB) on a major nucleolar phosphoprotein nucleolin

Nucleolin, a phosphorylated nucleolar protein, of 100 kDa selectively stained with bismuth tartrate and silver nitrate, is implicated in the transcription and maturation of pre-ribosomal RNA. Nucleolin also fulfills a structural function in nucleolar organization. Using immunocytochemistry the action of 5-6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), an inhibitor of hn/RNA synthesis known to modify the organization of the nucleolus, was studied for its effects on the distribution and the amount of nucleolin present. After DRB treatment, the morphology of the nucleolus was rapidly disturbed, but the distribution of the nucleolin remained unchanged: the dense fibrillar and the granular components were always positively immunostained. Thirty min after incubation with the drug, a strong increase of the amount of nucleolin occurred. Prolonged treatment led to a marked loss of label. Silver and bismuth staining showed that DRB does not seem to significantly affect the phosphorylation of nucleolin.