Analysis of the nucleotide sequence of the genetic locus for yeast mitochondrial RNA polymerase (RP047) reveals a continuous open reading frame with the coding potential for a polypeptide of 1351 amino acids, a size consistent with the electrophoretic mobility of this enzymatic activity. The transcription product from this gene spans the singular reading frame. In vivo transcript abundance reflects codon usage and growth under stringent conditions for mitochondrial biogenesis and function results in a several fold higher level of gene expression than growth under glucose repression. A comparison of the yeast mitochondrial RNA polymerase amino acid sequence to those of E. coli RNA polymerase subunits failed to demonstrate any regions of homology. Interestingly, the mitochondrial enzyme is highly homologous to the DNA-directed RNA polymerases of bacteriophages T3 and T7, especially in regions most highly conserved between the T3 and T7 enzymes themselves. introduction

Eukaryotic cells contain a physically and functionally distinct RNA polymerase activity for transcription of mitochondrial DNA (mtDNA). In the case of the yeast Saccharomyces cerevisiae, attempts to purify and size this enzyme suggest that the core catalytic activity is a monomeric polypeptide of approximately 145 kd (Kelly and Lehman, 1986). The single copy nuclear gene for yeast mitochondrial RNA (mtRNA) polymerase (RP047) has been mapped to chromosome VI, and disruption of this genetic locus results in a loss of functional mitochondria as defined by the petite phenotype (Greenleaf et al., 1986). Because of the central role of mtRNA polymerase in organelle function and biogenesis, we sought to analyze this genetic locus as a paradigm of nuclear genes encoding essential enzymatic activities involved in mtDNA replication and expression. The data reported here demonstrate that the gene for yeast mtRNA polymerase is orthodox with regard to its physical organization and basic transcriptional hallmarks; the overall amount of gene transcription correlates with demand for organelle biogenesis. A commonly held view is that mitochondria and chloropdasts have evolved their contemporary enzymatic activities from prokaryotic ancestral sources (Gray and Doolittle, 1982) and, as a consequence, their enzymes can be expected to bear amino acid homologies to cognate activities in E. coli. Such a relationship appears likely for the chloroplast DNA-encoded RNA polymer-