Recoding of the genetic code | The Genetic Code | Genetics, Biotechnology, Molecular Biology, Botany

⇒	The Genetic Code
⇒	Properties of genetic code
⇒	Chain initiation and chain termination codons
⇒	Synonym codons and degeneracy
⇒	Mutations and the genetic code
⇒	Wobble hypothesis
⇒	New genetic codes in mitochondria and ciliate protozoa
⇒	Suppressor mutations, base substitutions and suppressor tRNAs
⇒	Second genetic code, and second half of the genetic code
⇒	Recoding of the genetic code

Recoding of the Genetic Code
In recent years, examples have been discovered, where in a minority cases, there is a set of instructions (called 'reading signals') contained in the mRNA itself, that specify an alteration in how the genetic code is to be applied. These alterations are described as 'recoding' and include (i) alteration in the linear read out mechanism, involving + 1 or - 1 shift (or a hop of many nucleotides) in the frame of reading, at a particular point, when mRNA is being read (meaning of codon words remains unaltered); (ii) alteration in the meaning of the code words.

Alteration in the linear read-out frame
There are several examples of this recoding mechanism : (i) In E. coli, mRNA of RF2 (protein meant for termination of translation = release factor) programmes 30% of ribosomes to change to the + 1 reading frame after codon number 25 (codon 26 is UGA; see Fig. 30.11). The frameshift event is encouraged by an upstream sequence (termed 'stimulator') that pairs with 16S RNA in the ribosome. There are two components of recoding signals, a site of action (codons 25 and 26) and a stimulator, (ii) In a class of retroviruses, many mRNAs exhibit a ribosomal frame shift to the - 1 reading frame by tandem slippage of tRNA.

The two components of 'recoding signal' are, a site of action consisting of a heptanucleotide in the mRNA and a stimulatory sequence in the form of, stem loop or pseudoknot structure (present downstream; see Fig. 30.11). A similar heptanucleotide for frameshift is found in dna X gene of E. coli, where 50% of gene product is foreshortened due to frameshift. (iii) In higher animals, a protein called 'antizyme' renders the enzyme 'ornithine decarboxylase (ODC)' unstable. Decoding of antizyme mRNA requires a + 1 frameshift, which is regulated by the concentration of polyamines (product of ODC). (iv) In the mRNA for T4 gene 60, ribosomes hop a stretch of 50 nucleotides found between a pair of glycine codons. At the first glycine codon, 100% ribosomes are released; these ribosomes hop and read the second glycine codon downstream and continue translation of the rest of mRNA (Fig. 30.11). The stimulatory signals include the mRNA structure and the amino acid sequence of nascent peptide chain.

Fig. 30.11. Different alternatives leading to recoding of the genetic code.

Alteration in the meaning-of code words
In almost all organisms, there seem to be available 'recoding signals' in mRNAs leading to altered meaning of codons. These are exceptions to the universal character of genetic code. Following are some examples : (i) In some retroviruses and in some plant and bacterial viruses, the meaning of stop codon is altered to code for an amino acid leading to production of a fusion protein. For this purpose, in Moloney Murine LeukemiaVirus(MMLV), UAG codon is the site of action and a downstream pseudoknot is the stimulatory sequence. In atleast two bacterial genes and three mammaliar genes, an internal UGA codon codes for 'selenocysteine (Se Cys)', which has no unique codon in the genetic code dictionary. In mammals, the gene for selenoprotein 'P plasma protein' has more than 10 UGAs, 10 of them coding for Se Cys, but presence of UGA codon is not enough (some UGA codons in the same mRNA code for termination). The requirements for UGA to code for Se Cys include a specific minor tRNA, a specific 'elongation factor (EF)' and a particular 'downstream sequence' (sometimes this downstream sequence is 200 nucleotides away in the 3' non-coding region; see Fig. 30.11).





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