Tryptophan, codon read

Hence, we conclude that the higher molecular weight form of NT-3 was caused by reading through of the UGA stop codon by tryptophan incorporation. The UGA codon has been well documented to be leaky in early study on microbial molecular biology (9). Its use should be minimized in bacterial expression system. 

A surprise was encountered when the sequence of human mitochondrial DNA became known. Human mitochondria read UGA as a codon for tryptophan rather than as a stop signal (Table 5.5). Furthermore, AGA and AGG are read as stop signals rather than as codons for arginine, and AUA is read as a codon for methionine instead of isoleucine. 

Since nucleic acids generally cannot go in and out of mitochondria, all mitochondria appear to code for their own rRNAs and tRNAs. For the same reason, only the mRNAs that have been transcribed from the mitochondrial genome are translated in the mitochondria. A unique feature of mitochondrial mRNAs is the lack of a m G cap at the 5 end (reviewed by Bag, 1991). There are only 22-25 tRNA species in the mitochondria, indicating that a single tRNA can recognize more than one codon. There are some structural and sequence differences in the mitochondrial tRNAs. Furthermore, deviations from the standard genetic code, for example, utilization of AUA as the initiation codon instead of AUG, and reading UGA as a tryptophan instead of a stop codon, are a unique feature of mitochondria (Lapointe... 

As noted above, the genetic code used by cells is a triplet code, with every three-nucleotide sequence, or codon, being read from a specified starting point In the mRNA. Of the 64 possible codons in the genetic code, 61 specify individual amino acids and three are stop codons. Table 4-1 shows that most amino acids are encoded by more than one codon. Only two—methionine and tryptophan—have a single... 

Two amino acid residues, tryptophan and methionine, have unique codons—UGG and AUG, respectively. All other amino acids may be coded for by more than one codon, such that the code is said to be degeuerate. This degeneracy is not uniform, but varies according to the particular amino acids. For example, three amino acids (arginine, leucine, and serine) have six codons, five amino acids have four, isoleucine has three, and nine amino acids have two. The first two letters of each codon provide the primary determinant in the specificity. For example, the codons for amino acid valine are GUU, GUC, GUA, and GUG. The open reading frame of the mRNA, which extends from the AUG codon to the termination codon, establishes the protein that is to be synthesized. 

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