High resolution autoradiographical detection of RNA in the interchromatin granules of DRB-treated cells

Isolated rat liver cells were pulse-labelled with tritiated uridine and post-incubated in the presence of an excess of unlabelled uridine and of adenosine analog DRB (5-6-dichloro-1-beta-D-ribofuranosyl benzimidazole). Nuclear radioactivity was detected with high resolution autoradiography. A significant labelling of the interchromatin granules was revealed in these conditions. Pretreatments of cells with low doses of actinomycin D in order to preferentially inhibit ribosomal RNA (rRNA) synthesis prevented the labelling of the interchromatin granules during subsequent DRB treatments. These observations indicate that in DRB-treated cells, the interchromatin granules are sites of transfer or of accumulation of nucleolar RNA. Our results are discussed in connection with our knowledge of the action of DRB on RNA metabolism in mammalian cells and with recent data concerning the still enigmatic interchromatin granules which are present in the nuclei of most cells.