Saccharomyces cerevisiae coordinates accumulation of yeast ribosomal proteins by modulating mRNA splicing, translational initiation, and protein turnover.

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Abstract
The rate of accumulation of each ribosomal protein is carefully regulated by the yeast cell to provide the equimolar ratio necessary for the assembly of the ribosome. The mechanisms responsible for this regulation have been examined by introducing into the yeast cell extra copies of seven individual ribosomal protein genes carried on autonomously replicating plasmids. In each case studied the plasmid-borne gene was transcribed to the same degree as the genomic gene. Nevertheless, the cell maintained a balanced accumulation of ribosomal proteins, using a variety of methods other than transcription. (i) Several ribosomal proteins were synthesized in substantial excess. However, the excess ribosomal protein was rapidly degraded. (ii) The excess mRNA for two of the ribosomal protein genes was translated inefficiently. We provide evidence that this was due to inefficient initiation of translation. (iii) The transcripts derived from two of the ribosomal protein genes were spliced inefficiently, leading to an accumulation of precursor RNA. We present a model which proposes the autogenous regulation of mRNA splicing as a eucaryotic parallel of the autogenous regulation of mRNA translation in procaryotes. Finally, the accumulation of each ribosomal protein was regulated independently. In no instance did the presence of excess copies of the gene for one ribosomal protein affect the synthesis of another ribosomal protein.