How to Balance the Many Roles of tRNAs During the Creation of New Genetic Codes

Transfer RNA is a remarkable molecule, in that it sits at the crossroads between many competing functions, including charging, loading onto the ribosome, interacting with the ribosome for peptide bond formation. In all of these functions, it is competing with other tRNAs of similar shape and function, and must both be efficient in the production of proteins, but also highly specific in decoding the genetic code. For these reasons, tRNA molecules are likely under heavy selection pressure, and engineering them unsurprisingly can lead to large fitness costs for an organism. However, if we are to engineer novel genetic codes to improve the diversity of protein function, it will be essential to seamlessly integrate new tRNAs into these novel genetic codes. To this end, the Ellington lab has undertaken the engineering and directed evolution of tRNA molecules in the context of expanded genetic codes. We will present data on the fitness costs of an expanded 21 amino acid genetic code in which amber codons are decoded as nitrotyrosine, iodotyrosine, or selenocysteine and on the directed evolution of tRNA molecules to better accommodate the encoding of these new amino acids. Surprisingly, the efficient incorporation of the non‐canonical amino acids resulted in a reduction in the production of the tRNAs, and a concomitant catholicism towards modification. These counterintuitive results support the hypothesis that tRNA modifications by and large improve the specificity of protein translation. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .