Abstract
In eukaryotes the assembly and function of holocytochrome c oxidase requires the protein products of both nuclear and mitochondrial genes 1. Some of these genes provide protein subunits of the holoenzyme itself while others are required specifically for the biosynthesis or assembly of these subunits into a functional oligomer2. In the budding yeast Saccharomyces cerevisiae. three mitochondrial genes (COX1, COX2, and COX3) and six nuclear genes (COX4, COX5a, or COX5b, COX6, COX7, COX8, and COX9) encode the subunit polypeptides of the complex. COX5a and COX5b encode interchangeable isologs of subunit V, designated Va and Vb 3-5. All other subunits are specified by unique genes present in single copy on their respective genomes. Considering that nine structural genes in two different cellular compartments specify the subunit polypeptides of cytochrome c oxidase, the synthesis and assembly of the holoenzyme poses many interesting questions. Is the expression of this set of genes coordinated? If so, at what level (transcription, transcript processing, translation, or assembly) is their expression regulated? What is the nature of the intracellular effectors (metabolites, trans-acting proteins, etc.) that serve to modulate the level of each subunit and determine the overall level of holocytochrome c oxidase activity in cells? Are COX5a and COX5b expressed differentially and if so, do their protein products, Va and Vb, confer different properties on the holoenzyme?
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