CONTROL OF RIBOSOMAL RNA PROCESSING IN EUKARYOTES

ABSTRACT Processing of pre-rRNA plays a major role in eukaryotes. The control mechanisms involved seem to result in the fine adaptation of ribosome biogenesis to efficiency of translation. Experimental evidence on some of these control mechanisms is presented. Identification of 5′-terminal triphosphates of adenosine and guanosine in 37 S primary pre-rRNA of S.cerevisiae shows that it is the primary transcript of rRNA genes. These results suggest that processing of primary pre-rRNA can start only after transcription of rRNA genes is completed. Experiments on D-galactosamine block of RNA synthesis show that processing and nucleo-cytoplasmic transfer of rRNA in liver is independent of continuous transcription. Analysis of labelling kinetics data for liver pre-rRNA reveals considerable flexibility in the sequence of nuclease attacks on pre-rRNA chains and the simultaneous operation of several processing pathways. The cleavages leading to mature 28 S and 18 S rRNA appear to be under more stringent control than those generating intermediate pre-rRNA. Block of protein synthesis with low doses of cycloheximide does not inhibit transcription, but alters the processing pathways of pre-rRNA and creates a dichotomy in the output of large and small ribosomes. The role of the supply of ribosomal proteins in the control of pre-rRNA processing is emphasized.