Cox1 mutation abrogates need for Cox23 in cytochrome c oxidase biogenesis

Cox23 is a known conserved assembly factor for cytochrome c oxidase, although its role in cytochrome c oxidase (CcO) biogenesis remains unresolved. To gain additional insights into its role, we isolated spontaneous suppressors of the respiratory growth defect in cox23 Delta yeast cells. We recovered independent colonies that propagated on glycerol/lactate medium for cox23 Delta cells at 37 degrees C. We mapped these mutations to the mitochondrial genome and specifically to COX1 yielding an (IF)-F-101 substitution. The (IF)-F-101 Cox1 allele is a gain-of-function mutation enabling yeast to respire in the absence of Cox23. CcO subunit steady-state levels were restored with the (IF)-F-101 Cox1 suppressor mutation and oxygen consumption and CcO activity were likewise restored. Cells harboring the mitochondrial genome encoding (IF)-F-101 Cox1 were used to delete genes for other CcO assembly factors to test the specificity of the Cox1 mutation as a suppressor of cox23 Delta cells. The Cox1 mutant allele fails to support respiratory growth in yeast lacking Cox17, Cox19, Coa1, Coa2, Cox14 or Shy1, demonstrating its specific suppressor activity for cox23 Delta cells.