TRNA codon assignment

An important factor in the evolution of the genetic code is certainly provided by the aminoacyl-tRNA synthetases (see Sect. 5.3.2). It is clear that the two synthetase classes are not randomly distributed across the matrix of the amino acid assignment of the genetic code. For example, with one exception, all XUX codons code for class 1 synthetases, while all XCX codons code for class 2 aminoacyl-tRNA synthetases. A possible explanation could be that the synthetases and the genetic code evolved simultaneously. However, it is more likely that these enzymes evolved when the genetic code had already been established (Wetzel, 1995). 

The second key step for the expansion of the biosynthesizing system to introduce nonnatural amino acids is the expansion of the genetic codes. Schultz [38] and Chamberlin [39] first assigned an amber (UAG) stop codon to a nonnatural amino acid (aa ). By adding an aa -tRNA with a CUA anticodon as a suppressor of the amber codon, they successfully introduced the nonnatural amino acid at that position. Since then, the amber suppression method has been employed by a number of researchers. This method is advantageous in that an unsuccessful decoding of the UAG codon automatically leads to... 

C-labeled histidine-tRNA. Only the aminoacyl-tRNA whose binding is directed by the trinucleotide codon will become bound to the ribosomes and retained on the nitrocellulose filter. The amount of radioactivity retained by the filter is a measure of trinucleotide-directed binding of a particular labeled aminoacyl-tRNA by ribosomes. Use of this binding assay to test the 64 possible codon trinucleotides against the 20 different amino acids quickly enabled researchers to assign triplet code words to the individual amino acids. The genetic code was broken. 

The information contained in the base sequence of the mRNA template is interpreted in sequences of three bases called codons each codon represents one amino acid. Therefore, the unit of information is the codon. Since there are four major bases in mRNA, 4 (i.e. 64) different codons are possible. The 64 triplets constitute the genetic code (Table 17.1). All codons have been assigned to amino acids or punctuation signals. Three triplets (UAA, UAG and UGA) are not complemented by anticodons on tRNAs and serve to signal that the polypeptide chain has been completed. Of the other 61 triplets which have complementary tRNAs, two (AUG and GUG) have additional roles in the initiation of protein synthesis. Since there are only 20 amino acids, most amino acids are specified by more than one codon, i.e. the code is degenerate. The genetic code applies to prokaryotes and eukaryotic nuclear and chloroplast mRNAs but not to... 

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