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Distribution of U3 snRNA in the nucleolus of CHO cells

Localization of snRNA at the ultrastructural level was studied in the nucleolus of CHO cells by EM autoradiography. In conditions where snRNA U3 is the only RNA species labelled in the nucleolus, silver grains were largely found at the periphery, over the granular ribonucleoprotein component and the perinucleolar condensed chromatin; this enrichment was quantitatively significant. Inhibition of pre‐rRNA synthesis with actinomycin D did not alter the concentration or the distribution of U3 inside the nucleolus. The results are consistent with the demonstration that U3 is hydrogen‐bonded to 28S pre‐rRNA, and thus should be found in the granular compartment where 32S‐28S pre‐rRNA is assembled into 55s RNP.