18. The anticodon and/or the amino acid arms of a tRNA are key for a specific aminoacyl-tRNa synthetase to recognize This was revealed via: crystal structure determination of the synthetases complexed with their cognate tRNAS, sequence comparison of trnas binding to the same synthetase and mutagenesis studies When the anticodon of tRNAVal is changed from UaC to cau. the mutated tRNAVal can be recognized by Met-tRNa synthetase and generating a met-tRNAvVal
18. The anticodon and/or the amino acid arms of a tRNA are key for a specific aminoacyl-tRNA synthetase to recognize • This was revealed via: crystal structure determination of the synthetases complexed with their cognate tRNAs; sequence comparison of tRNAs binding to the same synthetase; and mutagenesis studies. • When the anticodon of tRNAVal is changed from UAC to CAU, the mutated tRNAVal can be recognized by Met-tRNA synthetase and generating a Met-tRNAVal
When the 3: 70 base pair on the amino acid arm of tRNacys is changed from from CG to G-U. the mutated tRNACys is able to carry Ala, instead of Cys a"microhelix' containing only 24 of the 76 nucleotides of tRNAAla is recognized and aminoacylated by Ala-tRNa synthetase
• When the 3:70 base pair on the amino acid arm of tRNACys is changed from from C.G to G.U, the mutated tRNACys is able to carry Ala, instead of Cys. • A “microhelix” containing only 24 of the 76 nucleotides of tRNAAla is recognized and aminoacylated by Ala-tRNA synthetase
3 Amino acid o arm Positions(green and 5 orange)of nucleotides involved in recognition TUC arm DHU arm between tRNAs and aminoacyl-tRNA synthetases Extra arm Anticodon arm Anticodon
Positions (green and orange) of nucleotides involved in recognition between tRNAs and aminoacyl-tRNA synthetases
76 A single g=u base pair is the only element needed for specific binding of tRNAAla and aminoacylation by Ala-tRNA synthetase G●U70 3 76 60 10 5 1 G●U70 30 40 66 Deleted nucleotides 10 (a) (b)
A single G=U base pair is the only element needed for specific binding of tRNAAla and aminoacylation by Ala-tRNA synthetase
19 Proofreading by some aminoacyl- tRNA synthetases increases the fidelity of protein synthesis The identity of the amino acid attached to a specific tRNa is not checked by the ribosome(ala-tRNaCyS experiment) The calculated rate of incorrect incorporation of val in place of lle is I in 200, but the observed is only I in 3000. Some synthetases are able to proofread the incorrectly incorporated similar amino acids(e.g, between Val and Ile) at the aminoacyl-AMP stage or the aminoacyl tRNA stage
19. Proofreading by some aminoacyltRNA synthetases increases the fidelity of protein synthesis • The identity of the amino acid attached to a specific tRNA is not checked by the ribosome (Ala-tRNACys experiment). • The calculated rate of incorrect incorporation of Val in place of Ile is 1 in 200, but the observed is only 1 in 3000. • Some synthetases are able to proofread the incorrectly incorporated similar amino acids (e.g., between Val and Ile) at the aminoacyl-AMP stage or the aminoacyltRNA stage