Conformational aspects and functions of tRNA.

In the process of protein biosynthesis, an aminoacyl-tRNA synthetase strictly recognizes the cognate amino acid and tRNA species. Spectroscopic and biochemical analyses have been made of a heterologous system of Thermus thermophilus glutamyl-tRNA synthetase and Escherichia coli tRNAGlu. The conformational difference between the initial complex and active complex has been observed, which is probably related with the strict recognition of aminoacyl-tRNA synthetase. tRNA species are post-transcriptionally modified at specific sites. Two types of modified uridine nucleosides have been found in the first position of anticodon, namely 5-hydroxyuridine derivatives (xo5U) and 5-methyl-2-thiouridine derivatives (xm5s2U). From the analyses of nuclear magnetic resonance spectra, the conformational characteristics of the two types of modified uridine nucleotides have been found to be remarkably different from each other. The conformational flexibility of xo5U nucleotides allow the multiple recognition of codons, whereas the conformational rigidity of xm5s2U nucleotides guarantees the recognition of correct codons. The modification of uridines in the first position of anticodon contributes to the correct and efficient translations of codons in protein biosynthesis.