Pre-ribosomal RNA is processed in permeabilised cells at the site of transcription

The available data concerning the subnucleolar localisation of the individu al steps of precursor-ribosomal RNA (pre-rRNA) processing are ambiguous. Ac cording to in situ hybridisation studies, the late steps of pre-rRNA proces sing have been located into the granular component of the nucleolus, but fa ctors engaged in these events were found being enriched in the dense fibril lar component. In this study, by utilisation of permeabilised human cells, we demonstrate that the newly synthesised, bromouridine-labelled pre-rRNAs reside at, or near, the sites of transcription. We provide evidence that pr ocessing of pre-rRNA occurs in permeabilised mammalian cells and that the i ncorporated bromouridine residues do not interfere with pre-rRNA maturation . Our results suggest that the maturation process of ribosomal RNA in perme abilised cells takes place at, or nearby, the site of transcription and tha t the processing complex is assembled during or early after the rRNA transc ription.