Saccharomyces cerevisiae Coordinates Accumulation of Yeast Ribosomal Proteins by Modulating mRNA Splicing, Translational Initiation, and Protein Turnover

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.

Yeast Ribosomal Protein L24 Affects the Kinetics of Protein Synthesis and Ribosomal Protein L39 Improves Translational Accuracy, While Mutants Lacking Both Remain Viable

“…Sucrose Gradient Analysis. This was performed as described by Warner et al (26) with some modifications. To a 200 mL cell culture in mid-log phase was added 100 µg/mL cycloheximide.…”

The Accumulation of Three Yeast Ribosomal Proteins under Conditions of Excess mRNA is Determined Primarily by Fast Protein Decay

“…When cloned S. cerevisiae r-protein genes became available, similar experiments were performed. As we observed with E. coli, an artificial increase in the copy number of some of these genes led to a higher level of mRNA, but the synthesis rate of the corresponding protein did not appear to increase to the same extent (14,25,36). This phenomenon was interpreted as a reflection of a translational control mechanism used to control the synthesis of r-proteins under natural conditions.…”

Nutritional and Growth Control of Ribosomal Protein mRNA and rRNA in Neurospora crassa

“…The regulatory factor(s) which binds to this region have not yet been identified, and consequently its role, if any, in coordinating transcription by RNA polymerases I and II is unknown. Although regulation of r-protein gene expression in yeast occurs primarily at the transcriptional level, post-transcriptional control mechanisms such as mRNA splicing (19) and protein degradation (20,21) are also operative in some cases.…”