Uda

Se selenocysteine

BOX 14-1 FIGURE 1 The structures of cysteine and selenocysteine.

the isolation of a mutant tRNA that carries a nucleotide substitution in the anticodon. Recall that tRNA synthetases frequently do not rely on interaction with the anticodon to recognize cognate tRNAs. Hence, a subset of tRNAs can bo mutated in their anticodons but still be charged with their usual cognate amino acids. As a consequence of the anticodon mutation, however, the mutant tRNA delivers its amino acid to the wrong codon. In other words, the ribosome and the auxiliary proteins that work in conjunction with the ribosome (which we will discuss shortly) primarily check that the charged tRNA makes a proper codon-anticodon interaction with the mRNA. The ribosome and these proteins do little to prevent an incorrectly charged tRNA from adding an inappropriate amino acid to the growing polypeptide.

A classic biochemical experiment nicely illustrates the point that the ribosome recognizes tRNA and not the amino acid that it is carrying. Consider the charged tRNA cys1einyl-tRNA(vs (remember that the prefix identifies the amino acid and the superscript identifies the nature of the tRNA). The cysteine attached to cysteinyI-tRNAc>s can be converted to an alanine by chemical reduction to give alanine-tRNACys (Figure 14-10). When added to a cell-free proiein-synthesizing system, alanine-tRNA^* introduces alanines at codons that specify insertion of cysteine.

Thus, the translation machinery relies oil the high fidelity of the aminoacyl tRNA synthetases to ensure the accurate decoding of each mRNA (see Box 14-1, Selenocysteine).

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