Unusual location of a mitochondrial gene - Subunit III of cytochrome c oxidase is encoded in the nucleus of chlamydomonad algae

The algae of the family Chlamydomonadaceae lack the gene cox3 that encodes subunit III of cytochrome c oxidase in their mitochondrial genomes. This ob servation has raised the question of whether this subunit is present in cyt ochrome c oxidase or whether the corresponding gene is located in the nucle us. Cytochrome c oxidase was isolated from the colorless chlamydomonad Poly tomella spp., and the existence of subunit III was established by immunoblo tting analysis with an antibody directed against Saccharomyces cerevisiae s ubunit III. Based partly upon the N-terminal sequence of this subunit, olig odeoxynucleotides were designed and used for polymerase chain reaction ampl ification, and the resulting product was used to screen a cDNA library of C hlamydomonas reinhardtii. The complete sequences of the cox3 cDNAs from Pol ytomella spp. and C. reinhardtii are reported. Evidence is provided that th e genes for cox3 are encoded by nuclear DNA, and the predicted polypeptides exhibit diminished physical constraints for import as compared with mitoch ondrial-DNA encoded homologs. This indicates that transfer of this gene to the nucleus occurred before Polytomella diverged from the photosynthetic Ch lamydomonas lineage and that this transfer may have occurred in all chlamyd omonad algae.