ISIS Report 01/09/04
Synthetic Genes in Food Crops
Prof. Joe Cummins explains why
genes inserted into GM crops are not "substantially equivalent" to genes in
their native state
A fully referenced version of
this article is posted on ISIS members’ website.
Details here.
In North America, genetically modified (GM) foods unlabeled and untested
are taking over the food supply. Such foods are promoted on the fiction that
the foreign genes – usually taken from bacteria and viruses - inserted
into GM crops are "substantially equivalent " to the natural genes.
In reality, the genes used to create GM crops are synthetic
approximations of the natural genes. They contain synthetic DNA sequences tuned
to maximize production of foreign proteins in the plant, such as toxins killing
insects or enzymes degrading herbicides, which also provide firm patent
protection on the GM crop. Synthetic genes are used because the genes actively
expressed in bacteria or humans are not very active in crop plants. There are
several ways to solve the problem.
The first is to adjust the DNA code to suit the ‘codon bias’
typical of the crop plant species into which genes from bacteria or mammals are
introduced.
The genetic code is made up of 64 three letter codons (triplets, or
code words) for twenty amino acids plus words for translation start and stop.
Some amino acid such as methionine (met) and tryptophan (tryp) have only one
codon, while arginine (arg), leucine (leu) and serine (ser) each have six
codons. The degeneracy of the code allows for alternative DNA sequences for a
single protein. The frequencies with which different codons are used vary
between groups of organisms, which is why genes from bacteria are poorly read
in higher plants (and vise versa). For optimum gene expression, the code for a
transgene often needs to be rewritten to achieve adequate performance. The
number of possible gene sequences that can code for a single protein is
staggering, it is estimated to be about five times ten to the 47th
power. That number is within three orders of magnitude of the number of atoms
making up earth and five times larger than the number of water molecules on
earth. In synthesizing the genes used in GM crops, say, in altering a
Bacillus thuringiensis (Bt) Cry toxin gene for plants, a table of
plant-preferred codons is used to substitute the plant preference for the
bacterial preference.
Sometimes it is necessary to substitute one or more of the amino acids
so that the final Cry toxin can function in the plant cell environment. As
plant genetic engineering has "advanced", the crucial active domains of toxins
and enzyme are defined and "improved" to such an extent that the original
source protein from living organisms is hardly recognizable.
A third modification of the transgene is in the regulatory sequences
(frequently referred to as cis elements) such as promoters, introns and
transcription termination signals, which are usually taken from higher plants
or their viral and bacterial pathogens. Synthetic promoters have also been
created, loosely based on the cauliflower mosaic virus (CaMV) commonly used in
plant genetic engineering.
The use of synthetic genes in food crops has not been taken into
account sufficiently in the regulatory approval of the food crops. In spite of
the obvious differences between the synthetic and the natural genes from which
they arose, regulators have allowed the genes and proteins produced in bacteria
to be considered appropriate surrogates in safety testing for the synthetic
genes and the proteins produced in food crops. This exposes the unhealthy
collusion of corporations and their regulators.
There seems to be a convenient fiction propagated by corporations,
government bureaucrats and academics who depend on grant money from
corporations and government, that genes from bacteria are used in producing
food crops or that genes from humans are used to produced plant
biopharmaceuticals, when, in fact, the genes used are synthetic approximations
to the real things. Even the courts seem to have accepted this convenient
fiction as fact.
The next generation of GM crops is evolving towards a minimal assembly
of active protein domains (domains are active area of proteins that serve as
signals for activates such as toxicity or enzyme function or environment
sensors for regulation) that are frequently patched together from a number of
different proteins. Safety testing is based, once again, on unreal surrogates
and the products are not labeled in the marketplace so that subtle changes
caused by a few amino acid changes or failure to heed secondary protein
modifications such as glycosylation will be difficult to trace as people are
adversely affected by consuming the synthetic products in GM crops.
It is imperative that the synthetic genes and their products be tested
thoroughly, not only for potentially toxic side effects but for stability and
recombination properties as well. These synthetic genes have not had an
evolutionary history and it is a major mistake to assume that the genes can be
expected to behave in all ways like the genes that they were built to
represent.
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