Accurate rybosomal protein synthesis depends on the correct base pairing between tRNA anticodons and 61 amino acids codons in mRNA. Because there are far fewer tRNAs than codons, a few tRNAs are restricted to their recognition of cognate codon, whereas other decode, besides of the canonical triplets, 1, 2 or even 3 codons. tRNA anticodon stem - loop sequences (ASLs) contain a number of conserved or semiconserved nucleosides, and they are rich with posttranscriptional modifications, especially at the "wooble position" (34 pos., first anticodon letter) and at 37 positions. It is well recognized that, status of modification of the both position strongly influences on tRNAs decoding capacity. In this communication we summarize results of model studies on an correlated impact of tRNA^Lys and tRNA^Val (E. Coli) modifications on their decoding ability (restriction versus amplification, respectively). The individual effect of the modifications on the tRNAs structure - function will be also dicussed.

Modified components of ASL Lys (mnm⁵s²U₃₄, t⁶A₃₇, Ψ₃₉) and ASL Val (cmo⁵U₃₄, m⁶A₃₇) were site - selectively incorporated (solid support synthesis) into ASLs sequences (heptadecamers). Mentioned above models were utilized for the coordinated system of biophysical/biochemical investigations, including studies on: crystal structures of the ternary complexes ASL (Lys, Val) - differently programmed ribosome 30S subunit (E. Coli); conformational and dynamic effects induced by the presence (or absence) of the modifications (NMR, molecular modeling); thermodynamic parameters of codon - anticodon binding on ribosome (A/P sites), and kinetics of the translocation A → P.

Basing on results of the mentioned above works, one can conclude that significant structural rearrangements must occur to attain biologically active, canonical anticodon arm conformation. These findings are also in agreement with recently published opinion that "an essential role of the modifications is to achieve kinetic and thermodynamic uniformity in translation process by individually "tuning" each tRNA seq., in order to overcome the inherent differences among
aa tRNAs" (O. C. Uhlenbeck, Mol. Cell, 2004, 16, 799 - 805).

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