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
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.
Alteration in the meaning-of code words
|Fig. 30.11. Different alternatives leading to recoding of the genetic code.
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
and a particular 'downstream sequence'
(sometimes this downstream sequence is 200 nucleotides away in the 3' non-coding region; see Fig. 30.11).