Glutamine codons

Curnow, A.W., Hong, K., Yuan, R., Kim, S., Martins, O., Winkler, W., Henkin, T.M., and SoU, D. 1997. Glu-tRNAGln amidotransferase A novel heterotrimeric enzyme reqnired for correct decoding of glutamine codons during translation. Proceedings of the National Academy of Science USA, 94 11819-26. 

RNA editing. A shortened version (apo B-48) of apolipoprotein B is formed by the intestine, whereas the full-length protein (apo B-lOO) is synthesized by the liver. A glutamine codon (CAA) is changed into a stop codon. Propose a simple mechanism for this change. 

In B. subtilis, the organism used for most studies, the cell contains approximately equivalent amounts of amidotransferase and tRNA° ", so that, potentially all available Glu-tRNA° " can bind to amidotransferase. Thus, the mechanism can provide a high level of control over the concentration of Gln-tRNA available for protein synthesis and, in addition, may provide a means of preserving the fidelity of glutamine insertion into protein by preventing the participation of the intermediate Glu-tRNA ", which responds also to the glutamine codons CpApA and CpApG [66], in protein synthesis. 

Variations in the code for cytoplasmic proteins have been found. In Tetrahymena and other ciliates the codon TAA represents glutamine rather than being a termination codon.375 A few proteins, including some in the human body, contain selenocysteine, the selenium-containing analog of cysteine. Selenocysteine is encoded by termination codon TGA. See Chapter 29 for details. However, even though TGA is occasionally used in this way, it serves as a termination codon for most proteins within the same cells.376 Thus, the context in which the codon TGA occurs determines how it is read by the ribosomal machinery. 

In intestinal epithelial cells the same apoB gene that is used to synthesize apoB-100 in the liver is used to make the shorter apoB-48 (48%) protein. This is accomplished in an unusual way that involves "editing" of the mRNA that is formed. Codon 2153 in the mRNA for the protein is CAA, encoding glutamine. However, the cytosine of the triplet is acted on by a deaminase, an editing enzyme, to form UAA, a chain termination codon.14 15 A third form of apoB is found... 

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