The yeast L(+)-lactate cytochrome c oxidoreductase or cytochrome b2 is a component of the mitochondrial intermembrane space. The protein is encoded by the nuclear genome, synthesized as a larger precursor in the cytoplasmic compartment, and then proteolytically processed to its mature form during its import into the mitochondria. The structural gene for yeast cytochrome b2 has been cloned. The complete nucleotide sequence of the gene with its 5' and 3' flanking regions was determined. The deduced primary structure of the cytochrome b2 precursor reveals an unusually long amino terminal extension of 80 amino acids. A variety of potentially significant sequences were identified in the region flanking the structural portion of the gene. Transcript mapping with both S1 nuclease and primer extension methods reveals that the site of RNA synthesis is 56-66 bp downstream from a putative TATA box. By Northern blot analysis and gene disruption, it is shown that there is only a single copy of the cytochrome b2 gene per haploid yeast nucleus. The cloned cytochrome b2 gene was used to probe specific mRNA levels and demonstrate that cytochrome b2 expression is transcriptionally repressed by glucose and induced by lactate. The inactivation of the chromosomal cytochrome b2 gene by integrative transformation led to a deficiency in L(+)-lactate dehydrogenase activity and consequently to the inability to use L(+)-lactate as a sole source of carbon. This is the first reported mutation affecting the structural gene of cytochrome b2.