Rare Codons
Background: The frequencies with which
the different codons appear in genes in E. coli are different from those
in genes derived from other organisms. The amount of specific tRNAs is
also reflected by the frequency of the codon, meaning that a tRNA which
recognizes a rarely used codon is present in low amounts. Therefore, various
genes that contain codons which are rare in E. coli may be inefficiently
expressed by this organism. Rare codons can cause premature termintion
of the synthesized protein or misincorporation of amino acids. Clusters
of rare codons stand a higher chance to create translation errors and reduce
the expression level. This problem can be solved either by exchanging codons
in the target gene for codons which are more frequently used in E. coli
[but see the further reading section], or,
alternatively, by co-production of the rare tRNAs [see
the experimental tips section].
A
table of the genetic code,
The
8 least used codons in E. coli, yeast, Drosophila, and primates,
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Experimental Tips
Certain bacterial strains were engineered to express higher
levels of some rare codons, i.e. Rosetta
(Novagen) or BL21
codon plus (Stratagene).
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Further Reading
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codons related material
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