Assembly of the mitochondrial membrane system. MRP1 and MRP2, two yeast nuclear genes coding for mitochondrial ribosomal proteins.

Nuclear respiratory deficient mutants of Saccharomyces cerevisiae impaired in mitochondrial protein synthesis have been screened for lesions in ribosomal protein constituents. Two mutants, each representative of a separate pet complementation group, have been analyzed. One of the mutants, E795, was found to have altered mitochondrial ribosomes as evidenced by the absence of some ribosomal proteins. The second mutant studied, C167, appeared to have more grossly altered ribosomes that could not be isolated by standard preparative procedures. In addition to being defective in mitochondrial protein synthesis, the mutants exhibit an absence of "a" and "b" type cytochromes, are partially blocked in processing of intron bI4 of the apocytochrome b gene, have reduced levels of mitochondrial 15 S rRNA, and convert to rho- and rho 0 mutants at a high frequency. The wild type genes MRP1 and MRP2 were cloned by transformation of the pet mutations in E795 and C167, respectively, with a recombinant plasmid library of wild type yeast genomic DNA. MRP1 codes for a basic protein of 37 kDa with no significant homology to any known prokaryotic or eukaryotic ribosomal protein. MRP2 codes for a 14-kDa polypeptide homologous to protein S14 of the Escherichia coli small ribosomal subunit and to a chloroplast-encoded component of chloroplast ribosomes. The levels of MRP1 and MRP2 mRNAs were examined in glucose-repressed cells and in cells undergoing adaptation to aerobic metabolism of ethanol. The steady state concentrations of the mRNAs increased during the first 3 h of derepression, indicating that expression of these mitochondrial ribosomal protein genes is transcriptionally regulated by glucose in a fashion analogous to respiratory carriers such as cytochrome c.