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Re: Recalling GMOs, Labelling and safety



(The following is quite technical)

Herve Le Meur wrote:

>By the way, is the final really the very same ?
>If Heine Deelstra is right, the gene should have been truncated. But
then
>how can it produce the right toxin ?
>HLM

Although I'm not sure about the exact sequence of the gene cloned into
corn, I do know that the expression of the full-length toxin was to
low to achieve pest resistance in plants other than tobacco (against
the tobacco hornworm) and tomato plants. Toxin levels were so low that
protection was not attained against less sensitive, but agronomically
important insect pests. Researchers then modified part of the Bt toxin
coding sequence so that it was efficiently expressed (and translated)
in plants. This was done by fusion of a synthetic* toxin gene for
amino acids 1-453 to the (natural) gene fragment encoding for amino
acids 454-615. The rest of the bacterial gene (amino acids 616-1178)
was not used. Expression of this gene in cotton plants showed that Bt
toxin levels were increased by 100 times and that Bt toxin constituted
0.02% of the protein in the plant. (For reference see Recombinant DNA,
2nd edition by James D. Watson et al. and (if your good in Dutch);
Moleculaire Biologie van Schimmels en Planten, 1998 by Prof. J.G.H
Wessels)

The Bacillus thuringiensis natural gene produces a large, inactive
pro-toxin that is about 1200 amino acids in length. This pro-toxin
releases upon digestion by proteases (in the insects gut) an active
68,000 Dalton fragment. So the pro-toxins of plants and B.
thuringiensis may differ in length,
while the active toxic fragment is exactly the same in size and mode
of action. Truncation of sequences before and after the 'toxic
fragment' might affect, due to folding differences, (1) the
crystallisation properties and (2) the susceptibility to proteases of
the
pro-toxin. The occurrence of (1) and/or (2) are not known to me.

*This fragment coded for the same amino acids as in the natural Bt
toxin gene, but it used codons preferred by plants.

I want to add that the data I have is not the most recent. Since I
joined this gentech list a few weeks ago I have been reading a lot
about these issues and I am trying to retrieve as many articles about
GE as I can. The information I do have however, together with careful
interpretation according to the education I receive(d) was the basis
for this message.

Heine J. Deelstra
==============================
<h.j.deelstra@bioledu.rug.nl>
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