Translation in a Box: Orthogonal Evolution in theSaccharomyces cerevisiaeMitochondrion

Abstract The ability to re-engineer and creatively evolve the translation system (TS) would allow invention of new coded polymers by altering the amino acid sidechain inventory and by shifting the polypeptide backbone into new chemical spaces. Unfortunately, the TS is difficult to manipulate and is more constrained over evolution than any other biological system. An orthogonal TS, running in parallel to the primary TS within a given host cell, would release constraints and allow TS manipulation. A fully orthogonal TS requires dedicated rRNAs, rProteins, aminoacyl-tRNA synthetases, and initiation and termination factors, none of which interact with the primary TS. The S. cerevisiae mitochondrial TS is fully orthogonal to the cytosolic TS. Mito-rRNAs, mito- rProteins, mito-tRNAs, mito-aminoacyl tRNA synthetases, and mito-translation factors are distinct from, physically separated from, and functionally independent of their cytosolic counterparts. Here, the S. cerevisiae mitochondrial translation system was subjected to various stresses including antibiotics, mutagenesis and truncation of mito-rProteins, or wholesale replacement of mito-rProteins. Directed evolution of these stressed systems was facilitated by controlled transitions between fermentation and respiration, by changing the carbon source in the growth medium; the dependence of S. cerevisiae survival on mitochondrial translation can be toggled on and off. Specific recreation of the resulting mutations recapitulate the evolved phenotypes. The method developed here appears to be a general approach for discovering functional dependencies. Suppressor mutations reveal functional dependencies within the S. cerevisiae mitochondrial TS. For example proteins Rrg9 or Mrx1 interact with the mito-TS and have critical role in its function. The combined results indicate that the S. cerevisiae mitochondrial TS can be engineered and evolved in isolation of the cytosolic TS. Significance The Central Dogma of Molecular Biology rules life on Earth. Information flows from DNA to mRNA to protein. In the last step of the Central Dogma, the translation system decodes mRNA and produces coded proteins by linking amino acids into polymers. Engineering and evolving the translation system could permits full technical control over this process and could lead to the generation of novel polymers. Here, we use the mitochondrial translation system in the budding yeast Saccharomyces cerevisiae for directed evolution of translation. We modify and evolve the translation system both directly and indirectly using antibiotics and gene editing tools and then measure resulting functionality. Our results show this secondary translation system inside S. cerevisiae mitochondria can be used as an approach for translation engineering.