| Abstract: | Cycloheximide given in vivo at low doses (2--5 mg/kg body weight) causes within 30 min a complete inhibition of protein synthesis in rat liver. The labelling of nuclear proteint is also strongly inhibited. Under these conditions, the amount of nucleolar 45-S pre-rRNA and its 14C]-orotate labelling remain unaffected for at least 4 h. These results show that initially the rates of synthesis and processing of 45-S pre-rRNA are not appreciably altered. On the other hand, drastic alterations in the 45-S pre-rRNA processing pathways occur at the early stages of cycloheximide action. Formation of 18-S rRNA is abolished and that of 28S rRNA is reduced to about half the level in control rats. This dichotomy in the production of the two ribosomal particles may be correlated with a block in the formation of 41-S and 21-S pre-rRNA. Generation of 36-S and 32-S pre-rRNA is still taking place, but the rate of their processing to nucleolar 28-S rRNA is decreased, thus causing the accumulation of these two pre-rRNA species. In parallel, processing of 45-S pre-rRNA to an aberrant 39-S rRNA species is markedly enhanced. The results obtained show that the channelling of nucleolar pre-rRNA along alternative processing pathways is under stringent control by the continuous supply of critical protein(s). |
