YtkA (CtaK) and YozB (CtaM) function in the biogenesis of cytochrome c oxidase in Bacillus subtilis

Cytochrome c oxidase in the respiratory chain of bacteria and mitochondria couples the reduction of molecular oxygen to form water with the generation of a transmembrane proton gradient. Bacillus subtilis has two heme A-containing heme-copper oxidases: the menaquinol oxidase cytochrome aa₃ and the cytochrome c oxidase cytochrome caa₃ . By screening three collections of mutants for defective cytochrome c oxidase, we found the genes for two, new membrane-bound assembly factors in B. subtilis: ytkA and yozB (renamed ctaK and ctaM, respectively). CtaK is a lipoprotein without sequence similarity to any protein of known function. We show that CtaK functions together with Sco1 (YpmQ) in a pathway, leading to the assembly of the Cu_A center in cytochrome caa₃ and seems to be a functional analogue to proteins of the periplasmic Cu_A chaperone family (PCu_A C). CtaM is required for the activity of both cytochrome caa₃ and cytochrome aa₃ and dispensable for the insertion of heme A into these oxidases. The orthologous Bacillus anthracis protein and the distantly related Staphylococcus aureus CtaM complemented CtaM deficiency in B. subtilis, establishing a common function of CtaM in these bacteria. As the overall result of our work, 12 different proteins are known to function in the biosynthesis of cytochrome c oxidase in B. subtilis.