Structure and Regulation of a Nuclear Gene in Saccharomyces Cerevisiae That Specifies MRP7, a Protein of the Large Subunit of the Mitochondrial Ribosome

The gene for MRP7, a 40-kilodalton protein of the large subunit of the yeast mitochondrial ribosome, was identified in a lambda gt11 expression library by immunological screening with a monoclonal antibody to MRP7. An intact copy of MRP7 was then isolated from a yeast genomic library by colony hybridization. Gene disruption showed that MRP7 protein was essential for ribosomal function. Sequencing of MRP7 revealed a coding region for a basic (pI 10.6), 43.2-kilodalton protein containing 371 amino acid residues. Amino acid residues 28 to 112 of the deduced MRP7 sequence aligned with the 84 residues of the Escherichia coli ribosomal protein L27, but no significant similarity was detected between the carboxy-terminal 259 amino acids of MRP7 and other protein sequences in existing computer data bases. Within the aligned region, there was 49% amino acid identity between MRP7 and L27, compared with the 57% identity observed between L27 and its homolog in Bacillus stearothermophilus. The steady-state levels of the MRP7 protein and its mRNA were monitored in response to catabolite repression and to increased dosage of the MRP7 gene. The response to catabolite repression was characterized by a ninefold change in the level of the protein and little, if any, change in the level of the mRNA. In cells carrying the MRP7 gene on a high-copy-number plasmid, the mRNA was increased 20-fold, but there was no significant increase in MRP7 protein. Furthermore, MRP7 mRNA and protein accumulated at normal levels in [rho0] cells, which are devoid of 21S rRNA, indicating that the protein is relatively stable in the absence of ribosome assembly. Together, these results suggest that MRP7 is regulated posttranscriptionally, probably at the level of protein synthesis rather than protein turnover.