Maturation and nucleo-cytoplasmic transport of rat-liver ribosomal RNA upon D-galactosamine inhibition of transcription.

D-Galactosamine (250 mg/kg body weight) 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 both nucleolar and nucleoplasmic genes is inhibited to the same extent. Under these conditions, prelabbeled 45-S pre-rRNA is processed quantitatively to nuclear 28-S and 18-S rRNA. The nucleocytoplasmic transport of both 28-S and 18-S rRNA remains unaltered for about 60 min after blockage of transcription. At this stage the nucleo-cytoplasmic transport of 18-S rRNA is almost completed. It is concluded that formation and nucleo-cytoplasmic transport of ribosomes is independent of concurrent transcription of rRNA or nucleoplasmic genes. At later stages, the nucleocytoplasmic transport of 28-S rRNA is delayed and its partial degaradation in the nucleus may take place. This effect is correlated with a decreased (up to 40% of controls) labelling of nuclear proteins. However, the labelling of total cellular or microsomal proteins remains unchanged up to 3 h after D-galactosamine administration. It is suggested that the last nuclear steps of ribosome formation are dependent on the continuous supply of rapidly-labelled nuclear proteins.