A molecular switch at the yeast mitoribosomal tunnel exit controls cytochrome <i>b</i> synthesis

Mitochondrial gene expression needs to be balanced with cytosolic translation to produce oxidative phosphorylation complexes. In yeast, translational feedback loops involving lowly expressed proteins called translational activators help to achieve this balance. Synthesis of cytochrome b (Cytb or COB), a core subunit of complex III in the respiratory chain, is controlled by three translational activators and the assembly factor Cbp3-Cbp6. However, the molecular interface between the COB translational feedback loop and complex III assembly is yet unknown. Here, using protein-proximity mapping combined with selective mitoribosome profiling, we reveal the components and dynamics of the molecular switch controlling COB translation. Specifically, we demonstrate that Mrx4, a previously uncharacterized ligand of the mitoribosomal polypeptide tunnel exit, interacts with either the assembly factor Cbp3-Cbp6 or with the translational activator Cbs2. These reciprocal interactions determine whether the translational activator complex with bound COB messenger RNA (mRNA) can interact with the mRNA channel exit on the small ribosomal subunit for translation initiation. Organization of the feedback loop at the tunnel exit therefore orchestrates mitochondrial translation with respiratory chain biogenesis.