Question

In an early translational experiment, a cysteine already attached to its tRNA was chemically converted to an alanine. These “hybrid” tRNA molecules were then added to a cell-free translation system from which the normal cysteine-tRNAs had been removed. When the resulting protein was analysed, it was found that alanine had been inserted at every point in the polypeptide chain where cysteine was supposed to be. Discuss what this experiment tells you about the role of aminoacyl-tRNA synthetases and ribosomes during the normal translation of the genetic code.

Answer 1

Answer :

The highly specific selection of tRNA substrates by aminoacyl-tRNA synthetases is an intriguing problem in RNA-protein recognition. Synthetases specific for each of the 20 amino acids encounter a pool of tRNAs in the cell having similar overall structures. The selection of the appropriate tRNAs for the attachment of each amino acid occurs by the formation of RNA-protein contacts unique to each cognate tRNA-synthetase pair.

Fidelity of transfer ribonucleic acid (tRNA) charging by amino acids ensures correct translation of the genetic code into proteins. Charging is catalysed by a set of enzymes known as aminoacyl-tRNA synthetases. Owing to the degeneracy of the genetic code, some of the different tRNAs have the same amino acid attached to them. Specificity of charging obeys universal rules and is ensured by positive elements, the identity determinants unique to each tRNA and responsible for its recognition by the cognate synthetase, and negative elements, the antideterminants that prevent false recognitions. To fulfil the aminoacylation specificity and prevent noncognate aminoacyl-tRNA delivery to the ribosome, some synthetases also mediate proofreading reactions that increase fidelity of the tRNA charging.

Each aminoacyl-tRNA synthetase has a binding site that recognizes a specific amino acid, and other binding areas that recognize a particular tRNA through unique identity sites at the acceptor stem and/or anticodon loop of the tRNA.

When there are different tRNAs for the same amino acid, the aminoacyl-tRNA synthetase that recognizes a specific amino acid must also recognize the set of tRNAs that can be charged with that amino acid.