Altered structure of ribosomal RNA transcription units in hamster cells after DRB treatment

The adenosine analog 5,6 dichloro-1-beta-ribofuranosyl benzimidazole (DRB) inhibits specifically mRNA production in eukaryotic cells, with concomitant alterations in pre-rRNA processing and nucleolar ultrastructure. We have focused our analysis on the effects of drug treatment on rRNA transcription through observations of spread ribosomal RNA (rRNA) transcription units from hamster cells. A significant increase in the size of ribosomal matrix units is paralleled by the appearance of abnormally large RNP structures containing elongating pre-rRNA. These structures correspond to nucleolar perichromatin granules which are known to appear in various conditions of cell growth when rRNA processing is altered. It is proposed that DRB-induced impairments in rRNA maturation are largely determined by alterations in the rRNA transcription process.