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Published online: 21
August 2005; | doi:10.1038/050815-13
Replacement found for bacterial DNA in transgenic
crops
Possible spread of antibiotic
resistance to gut bacteria squelched by using weed genes.
Roxanne
Khamsi
Scientists may have developed a potentially
less controversial way to bioengineer plants, by replacing a
marker gene normally borrowed from bacteria with a gene from
weeds. The new technique could make genetically modified crops
less contentious in places such as Europe, the team says.
Modern technology allows experts to
mix and match DNA from different organisms to enhance
favourable crop properties; a gene from fish, for example, can
make tomato plants frost resistant.
Most transgenic
crops also contain a bacterial gene, which helps researchers
distinguish between plants that have successfully picked up
foreign genes and those that haven't during crop development.
The two genes, one for the favourable trait and one for
antibiotic resistance, are tacked together and inserted into
seeds. When the growing plants are then doused with
antibiotic, those that haven't picked up the foreign genes die
off.
The marker gene typically comes from the
Escherichia coli bacterium. But critics of the
technology have pointed out that the code for antibiotic
resistance could hop, in a process known as horizontal gene
transfer, from the bioengineered food we eat into the bacteria
that live in our gut, thereby creating a superbug and a health
menace.
Gene for gene
Some
companies take an extra step to remove the
antibiotic-resistant gene before marketing their seeds. But
this doesn't always happen (see 'Stray seeds had antibiotic-resistance
genes').
Now Neal Stewart and Ayalew
Mentewab of the University of Tennessee in Knoxville,
Tennessee think they have a more foolproof way to eliminate
this threat, which involves scrapping the E. coli gene
and using one from a plant instead.
A gene called
Atwbc19 in the well-studied weed Arabidopsis
thaliana also confers antibiotic resistance; when this
gene is expressed at unusually high levels it helps to capture
and squelch antibiotic compounds.
Stewart and Mentewab
designed a piece of DNA including this gene and another that
codes for blueish pigments, making plants that pick it up
easily identifiable. The Atwbc19 gene is three times
larger than the antibiotic-resistance gene from bacteria. Both
the large size of the gene and the fact that it comes from
plants makes it less likely to hop into microbes, they
say.
Weeding out fears
To test whether the
Arabidopsis gene worked, they incorporated the linked
genes into tobacco plants; the tobacco seedlings with the
Arabidopsis gene continued to grow when blasted with
antibiotics. The results appear in the journal Nature
Biotechnology1.
Microbiologist Michael Syvanen
of the University of California, Davis agrees that the study
could calm fears about GM crops, particularly since the plant
gene simply can't be expressed by bacteria. "It produces a gene
which, if displaced by horizontal gene transfer back into bacteria,
would never be able to confer resistance to antibiotics," he
says.
The technique could
be adopted in parts of the world that have remained skeptical
about bioengineered foods, suggests Stewart. "There would be
some interest, especially in Europe, to move away from a
bacterial gene towards a plant gene," he says. But he cautions
that further testing is needed. Scientists must demonstrate,
for example, that the protein made from the weed gene has no
negative effect in humans.
References
- Mentewab A. & Stewart C. Nature Biotech.,
Advanced Online Publication. doi: 10.1038/nbt1134
(2005).
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