Abstract
We have examined expression of the protein coded within the MRP 1 locus of Saccharomyces cerevisiae. Direct evidence is provided for the assignment of the MRP1 gene product as a protein component of the small subunit of mitochondrial ribosomes. Further studies examined the extent to which the expression of the MRP1 protein is coordinated with the expression of other mitochondrial ribosomal components coded in the nuclear and mitochondrial genomes. Extra copies of the MRP1 gene were introduced into yeast cells to perturb expression from MRP1 relative to other mitochondrial ribosomal components to determine whether forms of regulation function to limit the accumulation of either MRP1 mRNA or protein under these conditions. Increases in MRP1 gene dosage were accompanied by substantial increases in both MRP1 mRNA and protein, indicating that their accumulation was not linked to the level of expression of other mitochondrial ribosomal components. This conclusion was confirmed by additional studies that showed that the accumulation of the MRP1 protein was unaffected in cells that did not express mitochondrially-encoded rRNAs. These results contrast with previous studies on the expression of two other mitochondrial ribosomal proteins indicating that regulatory properties of mitochondrial ribosomal proteins are quite diverse.
Highlights
Direct evidence is provided for the assignment of the MRPl gene product as a protein component of the small subunit of mitochondrial ribosomes
Extra copies of the MRPl gene were introduced into yeast cells to perturb expression from MRPl relative to other mitochondrial ribosomal components to determine whether forms of regulation function to limit the accumulation of either MRPl mRNA or protein under these conditions
Increases in MRPl gene dosage were accompanied by substantial increases in both MRPl mRNA and protein, indicating that their accumulation was not linked to the level of expression of other mitochondrial ribosomal components
Summary
Direct evidence is provided for the assignment of the MRPl gene product as a protein component of the small subunit of mitochondrial ribosomes. Expression of each of these genes is subject to catabolite repression as is the expression of the mitochondrially-encoded rRNAs [14] These observations indicate that both genomes respond in a coordinate fashion to changes in cell physiology that alter the demand for mitochondrial ribosomes. The accumulation of MRP7p, on the other hand, appears to be controlled at the level of protein synthesis [12] Whether this form of control is related to the autogenous feedback mechanism described for E. coli ribosomal proteins has not been determined. These results, given that only two proteins have been examined at the posttranscriptional level, indicate that the regulatory properties of mitochondrial ribosomal proteins may be quite diverse
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