Cbp3 and Cbp6 are dispensable for synthesis regulation of cytochrome in yeast mitochondria.

Cytochrome (Cyt) is the only mitochondrial encoded subunit from the complex. Cbp3 and Cbp6 are chaperones necessary for translation of the mRNA and Cyt hemylation. Here we demonstrate that their role in translation is dispensable in some laboratory strains, whereas their role in Cyt hemylation seems to be universally conserved. BY4742 yeast requires Cbp3 and Cbp6 for efficient mRNA translation, whereas the D273-10b strain synthesizes Cyt at wildtype levels in the absence of Cbp3 and Cbp6. Steady-state levels of Cyt are close to wildtype in mutant D273-10b cells, and Cyt forms non-functional, supercomplex-like species with cytochrome oxidase, in which at least core 1, cytochrome , and Rieske iron-sulfur subunits are present. We demonstrated that Cbp3 interacts with the mitochondrial ribosome and with the mRNA in both BY4742 and D273-10b strains. The polymorphism(s) causing the differential function of Cbp3, Cbp6, and the assembly feedback regulation of Cyt synthesis is of nuclear origin rather than mitochondrial, and Smt1, a mRNA-binding protein, does not seem to be involved in the observed differential phenotype. Our results indicate that the essential role of Cbp3 and Cbp6 is to assist Cyt hemylation and demonstrate that in the absence of heme , Cyt can form non-functional supercomplexes with cytochrome oxidase. Our observations support that an additional protein or proteins are involved in Cyt synthesis in some yeast strains.