Maturation and Nucleo-cytoplasmic Transport of Rat-Liver Ribosomal RNA upon D-Galactosamine Inhibition of Transcription

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Abstract
D-Galactosamine (250 mg/kg body wt) causes 90-95% inhibition of [14C]orotate or inorganic [32P]phosphate incorporation in vivo into rat liver nuclear RNA within 30 min. The transcription of nucleolar and nucleoplasmic genes is inhibited to the same extent. Under these conditions, prelabeled 45S pre-rRNA is processed quantitatively to nuclear 28S and 18S rRNA. The nucleocytoplasmic transport of both 28S and 18S rRNA remains unaltered for .apprx. 60 min after blockage of transcription. At this stage the nucleocytoplasmic transport of 18S rRNA is almost completed. Formation and nucleocytoplasmic transport of ribosomes is independent of concurrent transcription of rRNA or nucleoplasmic genes. At later stages, the nucleocytoplasmic transport of 28S rRNA is delayed and its partial degradation in the nucleus may take place. This effect is correlated with a decreased (up to 40% of controls) labeling of nuclear proteins. The labeling of total cellular or microsomal proteins remains unchanged up to 3 h after D-galactosamine administration. The last nuclear steps of ribosome formation are dependent on the continuous supply of rapidly-labeled nuclear proteins.