Cytochrome c oxidase (COX) comprises 13 subunits in mammals; three encoded in mitocondrial (mt) DNA and ten in nuclear DNA. Analysis of the evolution pattern of individual subunits can suggest tests of function and can serve as a model for the evolution of a multisubunit complex. In humans, the COX complex is unique among mammals in lacking the heart isoform of subunit VIII, which was apparently silenced sometime in primate evolution. The lineage in which it was lost will be identified, the promoter regions of COX8 heart and liver isoforms from closely related lineages will be isolated and characterized, and reporter constructs will be used to evaluate their tissue-specificity and activity in differentiated and undifferentiated myoblasts. COX7A isoform genes have mitochondrial targeting presequences which previous results suggest are involved in tissue-specific isoform function. This was unexpected because the presequences are removed during mitochondrial import and are thought not to participate in COX function. To investigate the potential for differential presequence function, the intracellular location of each of the two isoform proteins will be examined in cells that express both. Finally, the nucleotide substitution rates of three selected nuclear subunits will be determined.
COX provides a model system to test whether evolution of nuclear genes is parallel to that of mitochondrial genes with which they come in structural contact or functionally interact.
