Recognition Sites on tRNA

The function of tRNAs is to align the required amino acids according to the nucleotide sequences of mRNA. The tRNA molecule has four recognition sites to achieve this purpose.

(1) The amino acid attachment site which is the 3’ terminal –CCA sequences.

(2) The anticodon site consisting of the middle three bases on the anticodon loop which form the anticodon. The anticodon recognizes the three complementary bases which form the codon of mRNA.

(3) The ribosome recognition site which is common to all tRNAs and consists of the G-T-Ψ-C-R sequences on the TΨC loop.

(4) The amino acid activating enzyme recognition site (synthetase site) by which the activating enzyme recognizes and charges specific amino acids with tRNA.

Tertiary Structure. Electron density maps have revealed that tRNA has a tertiary structure. This structure is due to hydrogen bonds (i) between bases, (ii) between bases and ribose-phosphate backbone and (iii) between the backbone residues. (The hydrogen bonding in the double helical stem regions of the tRNA molecular are considered to be in the secondary structure).



Initiator tRNA. The starting amino acid in eukaryote protein synthesis n methionline while in prokaryotes it is N-formyl methionine. The tRNA molecules specific for these two amino acids are methionyl tRNA and N-formyl methionyl tRNA respectively. These tRNAs are called initiator tRNAs, because they initiate protein synthesis.



Specificity. Two important steps in translation during protein synthesis are the activation of amino acids and the transfer of amino acids to tRNAs. Each amino acid has a specific activating enzyme tRNA –aminoacyl synthetase. Thus there are 20 different tRNA aminoacyl synthetases for the 20 common amino acids found in proteins. Some tRNA synthetases can activate more than one amino acid, i.e., they show only a limited substrate specificity.





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