Modified Nucleosides and Codon Recognition+

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Abstract
This chapter focuses on the mechanisms by which the post-transcriptional modifications regulate the codon recognition of tRNAs, primarily from the viewpoint of conformational characteristics of modified nucleosides. Post-transcriptional modifications are heavily involved in the specificities of codon recognition and also of aminoacylation. Interestingly, the roles of the modifications in the anticodon are in many cases the altered conformational properties such as conformational "rigidity" and "flexibility," which directly result in the rigidity or flexibility in codon recognition, although the chemical structures of the modified nucleosides are so much different from each other. It is quite natural because the "wobble" of the base from the original location for the Watson-Crick base pair is essential for non-Watson-Crick base pairing. In the future, more direct structural studies should be done on the anticodon-codon recognition in the decoding center of the ribosome. In addition to such studies at the level of molecular structures, biological studies on the roles of posttranscriptional modifications are required. For example, Q is mostly conserved from bacteria to higher eukaryotes but is missing in tRNAs from Mycoplasma and mitochondria. If the Q modification is not indispensable for protein synthesis, it is a wonder that why many organisms have to have such a complicated hypermodification. The real biological role of Q may be to play an essential role in other unknown functions of tRNA. For answering this question, more biological approaches such as gene targeting of modification enzymes in mammalian systems appear to be important and are therefore in progress in laboratories.