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2-Plants: Replacement found for bacterial DNA in transgenic crops

-------------------------------- GENET-news -------------------------------

TITLE:  Replacement found for bacterial DNA in transgenic crops
SOURCE: Nature, UK, by Roxanne Khamsi
DATE:   21 Aug 2005

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Replacement found for bacterial DNA in transgenic crops

Possible spread of antibiotic resistance to gut bacteria squelched by
using weed genes.

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.

Top References
1 	Mentewab A. & Stewart C. Nature Biotech., Advanced Online Publication.
doi: 10.1038/nbt1134 (2005).


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