Amino Amber mutation

A number of triplets are referred to as nonsense codons because they are not known to code for any specific amino acid. When bacterial strains were developed in which mutations leading to the appearance of such triplets in their messenger were produced, 3 of the so-called nonsense codons, UAG, UGA (amber mutation), and UAA (ocre mutation) were found to act as punctuation codons for termination of the chain. 

Nonsense Mutation. A base substitution that mutates the codon of one amino acid to a stop codon, TAA (ochre), TAG (amber), or TGA (opal). A nonsense mutation results in protein truncation of a protein-coding gene. Thus, nonsense mutation is another type of severe mutation, which results in inactivation of the protein product most of the time. 

Nonsense mutation. A change in the base sequence that converts a sense codon (one that specifies an amino acid) to one that specifies a stop (a nonsense codon). There are three nonsense codons amber, ochre and something 1 forget (let me know if you read this - S. Mowbray). 

The nonsense snppression mutagenesis entails mutating a codon of interest to the nonsense stop codon, TAG (amber). This is followed by in vitro transcription of UAG-containing mRNA which directs the incorporation of modified/unnatural amino acid (m/u-amino acid) during in vitro or in vivo (by connecting mRNA and m/u-acyl tRNA) translation. The m/u-amino acid is aminoacy-lated onto the snppressor tRNA with anticodon, CUA which recognizes the amber codon on the mRNA. Adapted from Cornish and Schultz (1994)... 

Amber codon, nonsense codon the sequence UAG in mRNA. It does not code for any of the 20 proteogenic amino acids, and it results in the termination of protein synthesis (premature termination when UAG is produced by mutation of a sense codon). Potential precursors for the production of UAG by mutation are UCG (serine), UAU and UAC (tyrosine) and CAG (glutamine). 

Ochre codon the UAA sequence in mRNA. Like the amber codon, it signals the end of protein biosynthesis. The synthesized polypeptide chain is released after the incorporation of the amino acid encoded immediately before the 0.c. Ochre is probably the natural termination codon, and the one most widely employed by all living systems. See Ochre muttmts Ochre mutants bacterial mutants, which, as a result of a point mutation, possess a UAA codon in their mRNA (see Ochre codon). There are specific Suppressors (see) of ochre mutations. 

Suppressor W6th reference to Amber mutants (see) and Ochre mutants (see), a S. mutation is a secondary mutation in a tRNA dstron, which restores the ability of the tRNA to recognize the nonsense codon. The resulting new tRNA is called suppressor tRNA. The nonsense codon then becomes a sense codon and codes for a specific proteogenic amino add during translation. 

Some polar mutations of the lac operon have been found to be suppressed by other genes which are known to act as amber, ochre, or UGA suppressors. When z nonsense mutants are suppressed, an amino acid is inserted at the point of the mutation and the mutated enzyme activity is restored. The inserted amino acid has been found to be related to the class of suppressor employed. Suppressor genes produce mutated species of tRNA s which recognize nonsense codons, and insert amino acids at that point. For example the tyrosine tRNA of suppressor III recognizes the nonsense triplet UAG and inserts tyrosine on the growing peptide chain, thereby competing with the chain release reaction. 

To avoid competition between endogenous release factor and suppressor tRNA, a new strategy based on frameshift suppression mutations was developed. " " This strategy, based on the ability of tRNAs with extended anti-codons to suppress frameshift mutations, has two advantages over conventional suppression using the three-base amber, opal, and ochre stop codons competition between tRNA and release factors (RF) is avoided, and this approach allows more than one unnatural amino acid to be incorporated... 

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