Poster abstracts

Poster number 50 submitted by Noah Reynolds

Alternative mechanisms of determining the specificity of the genetic code in the cytoplasm and mitochondria.

Noah Reynolds (Department of Microbiology, The Ohio State University), Herve Roy (Department of Microbiology, The Ohio State University), Jiqiang Ling (Ohio State Biochemistry Program, The Ohio State University), Michael Ibba (Department of Microbiology and Ohio State Biochemistry Program, The Ohio State University)

Abstract:
The fidelity of translation is mainly determined by two events: synthesis of cognate amino acid:tRNA pairs by aminoacyl-tRNA synthetases (aaRSs) and accurate selection of aminoacyl-tRNAs (aa-tRNAs) by the ribosome. AaRSs define the genetic code by pairing tRNAs with the corresponding amino acids. To ensure faithful aa-tRNA synthesis, many aaRSs employ an additional editing activity that hydrolyzes incorrectly activated non-cognate amino acids. Eukaryotic organisms contain two distinct phenylalanyl-tRNA synthetases (PheRS), a cytoplasmic (ctPheRS) and a mitochondrial form (mtPheRS). CtPheRS is similar to the E. coli enzyme in that it consists of a heterotetramer made up of two alpha-beta heterodimers in which the alpha-subunits contain the active site and the beta-subunits contain the editing site. MtPheRS, as well as the chloroplast form, is an alpha-subunit monomer and lacks a recognizable editing domain, consistent with the absence of editing in some other mt aaRS. It has been shown that mtPheRS does not have the ability to edit mischarged tRNAPhe, nor do mitochondria contain any trans editing activity able to compensate for this deficiency. The two forms of PheRS differ both in their ability to edit Tyr-tRNAPhe and in the specificity of their amino acid binding sites. Amino acid activation kinetics show that mtPheRS is 180-fold more selective for Phe over Tyr than is ctPheRS. Even though mtPheRS does not edit, it will produce only one Tyr-tRNAPhe per ~8000 Phe-tRNAPhe synthesized when Phe and Tyr are present at comparable concentrations, as is the case under standard growth conditions. This error rate is close to the level of 10-4 usually cited for protein synthesis, and begs the question as to whether editing functions outside translation since PheRS can achieve adequate substrate specificity in its absence in the mitochondria.

Keywords: Aminoacyl-tRNA synthetase, Genetic code, Translation