Assembly of the mitochondrial membrane system. Characterization of the oxi2 transcript and localization of its promoter in Saccharomyces cerevisiae D273-10B.

The oxi2 gene of yeast mitochondrial DNA was previously shown to code for subunit 3 of cytochrome oxidase (Thalenfeld, B.E., and Tzagoloff, A. (1980) J. Biol. Chem. 255, 6173-6180). In Saccharomyces cerevisiae D273-10B, a 3.6-kilobase (kb) transcript has been mapped to the oxi2 region of mitochondrial DNA. This transcript, presumed to be the messenger RNA of subunit 3, has been characterized by Northern hybridization analysis and by S1 nuclease mapping. The 3.6-kb transcript has a 5' untranslated leader of 490 nucleotides followed by a 807-nucleotide long coding sequence and a 3' extension of approximately 2450 nucleotides. The nucleotide sequence of the coding region in the 3.6-kb transcript is identical with the gene sequence, thus excluding the presence of introns in the oxi2 gene. Analysis of mitochondrial RNA in cytoplasmic petite mutants containing the oxi2 gene, but with varying lengths of flanking sequences, suggest the presence of a common promoter for oxi2 and the upstream valine tRNA. The promoter has been mapped to a 400-nucleotide long region located on the 5' side of the tRNA gene. Generation of the mature subunit 3 mRNA must, therefore, involve the excision of the tRNA from the primary transcript.