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Regulation of ribosome formation in regenerating rat liver.

The rate of formation of the 45-S ribosomal precursor RNA in regenerating liver is approximately 50% higher at 19–26 h post-operative, when the rate of ribosome formation is high, than at 36–42 h post-operative, when the rate is returning to pre-operative levels. Although a 1-h treatment with cycloheximide or puromycin had no effect on the synthesis of the 45-S precursor, the processing to nuclear 18-S and 28-S RNA was reduced approximately 20% in both cases, indicating that a protein(s) involved in the processing (possibly) ribosomal proteins becomes limiting in the 42-h and even more so in the 19-h regenerating liver. Cycloheximide, which preserves the polyribosome structure, had no effect on the transport of ribosomal sub-units to the cytoplasm in the 42-h regenerating liver. In contrast, puromycin, which causes the conversion of these functional components to inactive monomers, inhibited transport by over 75%. The results indicate that ribosome formation in regenerating liver is subject to control at both the transcriptional and post-transcriptional levels.