The Ribosome Is Unable to Discriminate between Correctly and Incorrectly Charged tRNAs

The reason that so much responsibility falls on aminoacyl tRNA synthetases to ensure that the proper amino acid has been attached to the proper tRNA is that no further discrimination takes place after the charged tRNA is released from that enzyme. In other words, the ribosome "blindly" accepts any charged tRNA that exhibits a proper codon-anticodnn interaction, whether or not the tRNA carries its cognate amino acid.

This conclusion is supported by (wo kinds of experiments: one genetic: and the other biochemical. The genetic experiment involves

Box 14-1 Selenocysteine

Certain proteins, such as frie enzymes ┬┐Aitzrtfrcme percwidase and formate dehydrogenase, contain an unusual amino acrd called selenocysteine, which is pert of the catalytic center of the enzymes. Selenocysteine contains the trace element selenium in place of the sulfur atom of cysteine (Box 14-1 Figure 1). Interestingly, selenocysteine is not incorporated into proteins by chemical modification after translatiixi (as is true for certain ether unusual amino acids, such as hydmxyproline, which ft found in collagen). Instead, selenocysteine is generated enjymatrcaUy from sonne carried on a special tRNA that is charged by serine-tRNA synthetase This altered tRNA is used to incorporate sefenn cysteine directly into enzymes such as glutathione peroxidase as they are synthesized A dedicated (EF-Tu-like, see below) translation elongation factor delivers selenocysteinyl-tRNA to the nbosome at a codon (UGA) that would normally be recognized as a stop codon. Incorporation of selenocysteine at UGA cudons veep^es the presence of a special sequence element elsewhere in the mRNA. Thus, sdenocysteine can be thought of as a 2lst amino add that is incorporated into proteins by a modification ci the standard translation machinery of the cell.

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