Editing of tRNA for Structure and Function

Abstract

Abstract The degeneracy of the genetic code is implied in the need for 61 sense codons to specify 20 different amino acids; with the exception of methionine and tryptophan, each amino acid is encoded by more than one codon. This discrepancy between codon and amino acid numbers was first explained by Crick’s wobble hypothesis, which invoked the need for base-pairing flexibility between the first anticodon and third codon positions during decoding. Since the inception of the wobble rules, over 100 posttranscriptional modifications have been described, with the largest number affecting the anticodon of tRNA. As anticodon modifications accrue, new findings lead to a constant reinterpretation of the wobble rules to include novel effects on tRNA function. In general, anticodon modifications play key roles in translational fidelity and efficiency. However, anticodon-sequence alterations to a particular tRNA that permit decoding of multiple codons are part of a growing number of posttranscriptional changes collectively known as tRNA editing. In fact, the decoding changes imparted by tRNA editing provide a mechanism to effectively accommodate genetic code degeneracy. Although a number of editing events have direct effects in expanding a tRNA’s decoding capacity, some editing events indirectly affect tRNA function by repairing otherwise non-functional tRNAs. This chapter will attempt to summarize what is currently known about both types of tRNA editing in various organisms, with the proviso that due to the serendipitous nature of editing discoveries, the work presented here will undoubtedly not be conclusive. This chapter will rather compile the few existing examples of tRNA editing, and whenever possible will try to illustrate current efforts to characterize the different tRNA editing enzymes and the various mechanisms.