Asparagine codons

Ohama, T., Osawa, S., Watanabe, K. and Jukes, T.H. (1990) Evolution of the mitochondrial genetic code. IV. AAA as an asparagine codon in some animal mitochondria. Journal of Molecular Evolution 30, 329-332. 

The codons of the mRNA on the ribosomes are read from the 5 to the 3 end. Thus the synthetic polynucleotide (5 )A-A-A-(A-A-A) -A-A-C(3 ) contains the code for lysine (A-A-A) and asparagine (A-A-C) the actual polypeptide obtained using this mRNA in a cell-free system was Lys-(Lys),-Asn, and not Asn-(Lys) -Lys. 

Because asparagine was the carboxyl-terminal residue, we can conclude that the codon AAC was the last to be read. 

C3 transferase is a specific inhibitor of the Rho GTPase and has been used in lymphocytes, neutrophils, neuronal cells, epithelial cells and platelets as well as fibroblasts. Although all members of the Rho family of GTPases contain an asparagine residue at codon 41, only Rho seems to be a substrate for C3 transferase. However, many members of the Rho family have yet to be characterized, and it is possible therefore that other substrates for C3 transferase will be identified. 

It follows that the array of codons represents the instructions for producing 20 amino acids, the so-called common amino acids. These amino acids are named and symbolized as alanine (Ala), arginine (Arg), asparagine (Asn), aspartate (Asp), cysteine (Cys), glutamine (Gin), glutamate (Glu), glycine (Gly), histamine (His), isoleucine (lie), leucine (Leu), lysine (Lys), methionine (Met), phenylalanine (Phe), proline (Pro), serine (Ser), threonine (Thr), tryptophan (Trp), tyrosine (Tyr), and valine (Val). 

Fig. 2. A portion of the tandem mass spectrum of the glycopeptide, residues 195-204, containing the second glycosylation site, after treatment with PNGase E The mass difference of 115 Da between the b2 and bs ions is attributable to aspartic acid as the third residue (codon 197) rather than asparagine. (Reproduced with permission from Baldwin, MA. et al. [1993]. Trends in Analytical Chemistry 12, 239-248. Copyright Elsevier Science.)...

Viral replication in the presence of nelfinavir selects for drug resistance. The primary nelfinavir resistance mutation is unique to this drug and occurs at HIV protease codon 30 (aspartic acid-to-asparagine substitution) this mutation results in a sevenfold decrease in susceptibility. Isolates with only this mutation retain full sensitivity to other HIV protease inhibitors. Less conunonly, a primary resistance mutation occurs at position 90, which can confer crossresistance. In addition, secondary resistance mutations can accumulate at codons 35, 36, 46, 71, 77, 88, and 90, and these are associated with further resistance to nelfinavir, as well as cross-resistance to other HIV protease inhibitors. 

Because of the considerations just made, it is of interest to divide amino acids (see Fig. 10.3) into three classes (i) those that only contain G and/or C in the first and second positions of their codons (ii) those that only contain A and/or T and (iii) those that contain G and/or C as well as A and/or T. The GC class comprises four amino acids, alanine, arginine (quartet codons), glycine, and proline the AT class comprises seven amino acids, asparagine, isoleucine, leucine (duet codons), lysine, methionine, phenylala-... 

The alteration of a single nucleotide in the DNA directing the synthesis of these codons could lead to the substitution of a serine for an asparagine in a polypeptide. The alteration of a single nucleotide in the DNA directing the synthesis of these codons would necessarily lead to an amino acid substitution in the encoded... 

Such a polynucleotide chain theoretically can stimulate the incorporation of five lysines and one asparagine in the acid-precipitable fractions. In fact, it was observed that only lysine and asparagine incorporation take place, and the incorporated asparagine is always found in the N-terminal position. These initial experiments were extensively confirmed with more sophisticated methods also involving synthetic polynucleotides. The unidirectional reading of the N-terminal amino acid implies that the 5 -phosphate either coincides with or is very close to the codon for the N-terminal amino acid. 

Figure 25.16 illustrates this process of protein biosynthesis. It shows a tRNA molecule about to add to the mRNA codon AAU. Note that the tRNA molecule has a triplet base sequence TTA at one end of the molecule and the amino add asparagine (Asn) attached to the otho- end. The triplet TTA, called an anticodon, is the complementary base sequence to the codon AAU. The tRNA anticodon pairs up with the mRNA codon. The amino acid asparagine is then transferred to the end of the protein chain, where it links to valine (Val). Once the amino add has been transferred, the tRNA molecule is released from mRNA. 

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