GENTECH archive


Re: Bt corn

At Friday 21 May 1999  Herv Le Meur wrote:

>I'll risk myself in a topic that's not mine, but I guess that some
one will
>correct me if I'm wrong :
>Some codons give the same aminoacid in the protein produced by a
>It happens that some species have some codons preferred to code for
an aminoacid
>So, if you take the sequence of bp in Bt,
>and that you insert it in corn, you may perfectly well not have
enough quantity.
>It seems that one do not know why (maybe some on this list ?).

The basis of codon preference is the difference in tRNA concentrations
of the equivalent (resulting in  the same aminoacid) anti-codons
(='mirror-image' of a codon). When a ribosome starts translating a
messenger RNA (mRNA) it reads the sequence in pairs of three bases
(=codon). The 'docking' of a complementary tRNA to the codon is then
required to (1) incorporate the corresponding aminoacid into the
growing protein and
(2) continue with the synthesis. If a tRNA is rare, the ribosome
stalls, waiting for the tRNA to arrive. This will then become the rate
limiting step in the synthesis of the foreign protein. If it is
an overexpressed gene like Bt (behind very strong opine promoters)
ribosomes will be blocked and the organism will probably grow very
slow and can
even die.

>Then, a "simple" modification consists in replacing some codons
unusual in corn
>by other codons more usual in corn (but less in the bacterium) that
>will give the very same aminoacid.

Example: for the aminoacid leucines several codons exist (CUU, CUC,
CUA, CUG). If in a hypothetical organism the concentration of
CUG-complementary tRNA is low (and e.g. CUU is high), proteins
containing CUG encoded leucines will be formed in lower than expected
concentrations. This can be corrected by changing  the third base of
CUG to a U -> the codon becomes CUU and the protein will be formed in
higher concentrations.

>So the toxin at the end might be the same with a different coding
>Of course, I do not speak of the promoter, nor of the terminator (no
>with Terminator).

Promoters and terminators are nor transcribed, nor translated so they
don't show up in the mature protein.

>Any correction ?
Basically not.

Heine Deelstra