MOLECULAR-CLONING AND ANALYSIS OF THE NUCLEAR GENE MRP-L6 CODING FOR A PUTATIVE MITOCHONDRIAL RIBOSOMAL-PROTEIN FROM SACCHAROMYCES-CEREVISIAE

The Saccharomyces cerevisiae nuclear gene MRP-L6 was cloned by complem entation of the respiratory-deficient mutant pet-ts 2523 with a librar y of wild-type yeast genomic DNA. The isolated gene was part of a 3.8- kb sequenced DNA fragment containing, in addition to MRP-L6, two unass igned reading frames, ORF1 and ORF2. MRP-L6 codes for a basic protein of 205 amino acids and a molecular mass of 22.8 kDa. The protein exhib its significant sequence similarity to the ribosomal protein L6 of bac teria and chloroplasts. Unlike the corresponding bacterial proteins, h owever, the MRP-L6 protein (MRP-L6p) contains at its N-terminus a 16 a mino-acid leader sequence exhibiting the known characteristics of mito chondrial import signals. Disruption of MRP-L6 leads to the phenotype of a mitochondrial translation-defective, rho-negative yeast mutant. T he results are consistent with MRP-L6p representing an essential compo nent of yeast mitochondrial ribosomes. Expression of MRP-L6 was examin ed, under conditions of glucose repression and derepression, in wild-t ype cells and in a series of catabolite repression-defective yeast mut ants. In most cases, a distinct though small influence of the carbon s ource on the expression of an MRP-L6/lacZ reporter construct was obser ved.