Subunit II of Cytochrome c Oxidase in Chlamydomonad Algae Is a Heterodimer Encoded by Two Independent Nuclear Genes*

The mitochondrial genomes of Chlamydomonad algae lack the cox2 gene that encodes the essential subunit COX II of cytochrome c oxidase. COX II is normally a single polypeptide encoded by a single mitochondrial gene. In this work we cloned two nuclear genes encod-ing COX II from both Chlamydomonas reinhardtii and Polytomella sp. The cox2a gene encodes a protein, COX IIA, corresponding to the N-terminal portion of subunit II of cytochrome c oxidase, and the cox2b gene encodes COX IIB, corresponding to the C-terminal region. The cox2a and cox2b genes are located in the nucleus and are independently transcribed into mRNAs that are trans-lated into separate polypeptides. These two proteins as-semble with other cytochrome c oxidase subunits in the inner mitochondrial membrane to form the matur