A NEW POINT MUTATION IN THE NUCLEAR GENE OF YEAST MITOCHONDRIAL RNA-POLYMERASE, RP041, IDENTIFIES A FUNCTIONALLY IMPORTANT AMINO-ACID RESIDUE IN A PROTEIN REGION CONSERVED AMONG MITOCHONDRIAL CORE ENZYMES

The core enzyme of mitochondrial RNA polymerase in yeast is homologous to those of bacteriophages T3, T7 and SP6. In previous studies the id entification of the first conditional yeast mutant for this enzyme hel ped to identify the corresponding specificity factor and to elucidate their interaction inside mitochondria. In the present study we report the identification of a second nuclear mutation located in the gene fo r mitochondrial RNA polymerase. A comparison of the two temperature-se nsitive mutants demonstrates that the new mutant has a phenotype disti nct from the first one and characterizes a new important domain of the enzyme. Two different suppressor genes which both rescue the first mu tant do not abolish the defect of the second one and, in addition, an extremely high instability of mitochondrial genomes is observed in the new mutant. The enzymatic defect is caused by a single nucleotide exc hange which results in the replacement of the serine(938) residue by p henylalanine. This amino acid is located in the middle part of the pro tein in an as yet poorly characterized region that is not highly conse rved between mitochondrial core enzymes and bacteriophage-type RNA pol ymerases. However, the affected amino acid and the respective protein domain are specific for mitochondrial RNA polymerase core enzymes and may help to define enzymatic functions specific for the mitochondrial transcription apparatus.