We demonstrate here that mitoribosomal protein synthesis, responsible for the synthesis of oxidative phosphorylation (OXPHOS) subunits encoded by mitochondrial genome, occurs at high levels during glycolysis fermentation and in a manner uncoupled from OXPHOS complex assembly regulation. Furthermore, we provide evidence that the mitospecific domain of Mrp7 (bL27), a mitoribosomal component, is required to maintain mitochondrial protein synthesis during fermentation, but is not required under respiration growth conditions. Maintaining mitotranslation under high glucose fermentation conditions also involves Mam33 (p32/gC1qR homolog), a binding partner of Mrp7’s mitospecific domain, and together they confer a competitive advantage for a cell's ability to adapt to respiration-based metabolism when glucose becomes limiting. Furthermore, our findings support that the mitoribosome, and specifically the central protuberance (CP) region, may be differentially regulated and/or assembled, under the different metabolic conditions of fermentation and respiration. Based on our findings, we propose the purpose of mitotranslation is not limited to the assembly of OXPHOS complexes, but also plays a role in mitochondrial signaling critical for switching cellular metabolism from a glycolysis- to a respiratory-based state.

The extreme N-terminal residues of the mitochondrial ribosomal bL27m proteins reside within the ribosomal peptidyl transferase center (PTC) and are conserved from their bacterial ancestors. Mutation or truncation of the N-terminal region of the yeast Mrp7/bL27m protein did not inhibit protein synthesis but significantly impacted the efficacy of the mitochondrial translational process with respect to yielding proteins competent to assemble into functional oxidative phosphorylation enzymes. The requirement for the N-terminal residues of Mrp7/bL27m to support normal mitotranslation was more apparent under respiratory growth. We demonstrate that the N-terminal region of Mrp7/bL27m impacts the environment of the PTC and speculate the bL27m proteins serve to fine-tune and optimize mitoribosomal activity with respect to the downstream fate of the nascent chain.