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.The ribosome is undoubtedly the most thoroughly understood organelle of the cell (2, 37, 38). It has two subunits. The smaller is composed of one RNA molecule and 20 to 30 proteins; the larger is composed of two RNA molecules and 35 to 50 proteins. With only one or two exceptions (e.g., L7/L12), there is a single molecule of each comnonent in each Escherichia coli ribosome (11) and there is no reason to believe that eucaryotic ribosomes will be different.Our interest is centered on the synthesis of the ribosome, in particular, on the mechanisms used by the cell to ensure an adequate supply of each component used for assembly without needless accumulation of excess components (7,36). In short, we view efficient synthesis of ribosomes as a problem of molecular inventory control. That the cell can carry out this process effectively was apparent from earlier studies on both RNA (33) and proteins (9) showing that no more than 10% excess of any ribosomal component was synthesized. Since the assembly of ribosomes is presumably limited by the availability of the least abundant component, there is little margin for error.

Complex Regulation of the Yeast Heat Shock Transcription Factor

“…The growth rate dependence of protein synthesis is also reflected in changes in the concentration of rRNA and ribosomal proteins (r-proteins). Because r-proteins are synthesized in excess of what can be assembled into ribosomes (Warner et al, 1985;Maicas et al, 1988), r-proteins represent a major class of short-lived proteins, with half-lives of 1-5 min (Warner, 1989). Therefore, r-proteins represent a second major class of Ssb1/2p substrates.…”

“…In E. coli, some of this regulation is achieved by transcriptional attenuation and translational regulation via binding of ribosomal proteins to the RNAs of target operons (Freedman et al, 1985;Cole and Nomura, 1986). In yeast, regulation is partly transcriptional, via RAP1 and its binding site, the upstream activating sequence (UAS) rpg (Herruer et al, 1987;Moehle and Hinnebusch, 1991;Kraakman et al, 1993), although much of the regulation of ribosomal protein abundance is achieved by a competition between the assembly into ribosomes and the rapid degradation of unassembled ribosomal proteins (Warner et al, 1985;Maicas et al, 1988).Growth rate control clearly modulates the level of the translational machinery, which in turn influences the overall rate of protein synthesis. Changes in the rates of protein synthesis must, in turn, have profound implications for the protein chaperone system.…”

Structure and expression of the Saccharomyces cerevisiae CRY1 gene: a highly conserved ribosomal protein gene.

“…The regulation of expression of the yeast ribosomal protein genes that do not contain introns (e.g., RP39A and RP39B) is seemingly identical to that of the intron-containing ribosomal protein genes during heat shock, sporulation, or the stringent response or after a carbon source shift (11,29,33; M. Rotenberg and J. Woolford, unpublished data). However, recent data indicate that expression of several ribosomal protein genes during steadystate growth of yeast cells may be regulated in part at the level of mRNA processing (71). Further experiments will be necessary to elucidate the role, if any, of the short 5' exons or the introns or both in the regulation of ribosomal protein gene expression.…”

Ribosome stoichiometry: from form to function.

“…This poor correlation may reflect many posttranscriptional mechanisms, such as protein degradation. Indeed, when RPL3 mRNAs is transcribed 7.5 times as much as in wildtype cells, the RPL3 level increases by less than 20% [50][51][52][53]. A particularly intriguing mechanism could be that some RPs, when incorporated into ribosomes, inhibit the translation of their own mRNAs, thus providing an efficient functional feedback-loop that buffers mRNA variability.…”