A TEAM of scientists from Texas and Canada has found an enzyme produced
by soil bacteria that will break down toxins in pesticides and chemical
weapons. The researchers have also used ‘jumping genes’, pieces of DNA that
will hop between species, to persuade the fruit fly Drosophila to adopt
the gene that codes for the enzyme from the soil bacteria. This has allowed
the scientists to produce what they believe is the world’s first batch of
flies with an increased resistance to insecticides.
Farmers might one day use this resistance to protect beneficial insects
from the insecticides used to kill harmful insects. The researchers also
believe that their work could provide a clue to the speed with which insects
in the wild develop resistance to common insecticides, a question which
has baffled geneticists for many years.
The research has already attracted attention from the US Army, which
has funded some of the work. It is interested in methods of controlling
toxic nerve agents of the kind allegedly used by Iraq in the Gulf War. The
Army’s chemical research engineering laboratories, in Maryland, have tested
the enzyme and shown that it is able to break down some organophosphorous
nerve agents.
Several companies are also interested in exploiting the possibilities
of the enzyme, for purging the environment of residues of organophosphorous
toxins used in pesticides. Current procedures involve burning the compounds
or attacking them with chemicals to degrade them, which can leave toxic
products. If biological processes could degrade the compounds, the problem
would be avoided.
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The team will publish a paper on purifying the enzyme, called organophosphorous
anhydrase, in the Journal of Bacteriology in December. The researchers have
submitted their work on the transgenic enzyme for publication in the Proceedings
of the National Academy of Sciences in the US.
The first step in purifying a sizeable quantity of the enzyme was to
isolate the gene that codes for the enzyme from a sequence of DNA known
as plasmid, found in the soil bacteria Pseudomonas diminuta. The researchers
have not yet established why this gene is important to the bacteria. James
Wild, a geneticist at Texas A&M University, identified and purified
the gene. Next he inserted it into a smaller plasmid using a ‘promoter’,
a sequence of DNA which indicates to a cell the start and finish of the
gene.
The team then slipped this smaller plasmid into Escherichia coli bacteria,
using another promoter, called tac. The E coli did not produce as much of
the protein as the team had hoped, so they transferred the gene into DNA
in a culture of insect tissue, derived from caterpillars called fall armyworms.
They used modified insect virus to transport the gene into the cells of
the insect tissue culture, and these live cells produced the enzyme in large
quantities.
Wild and Frank Raushel, a specialist in the mechanics of enzymes at
the university, found that unlike most enzymes, organophosphorous anhydrase
will attack the internal bonds of a very wide range of pesticides. Most
enzymes are highly selective. These include: parathion, a pesticide used
to control garden insects; diazinon, which controls fire ants; and coumaphos,
used to control the ticks that cause cattle fever. Once the internal bonds
of these compounds are broken, the pesticides degrade and lose their toxicity.
Once the researchers had shown that they could produce the enzyme in
a tissue culture, they decided to try to express the gene in an insect.
John Phillips at the University of Guelph in Canada took the original gene
from the soil bacteria and attached it to a ‘heat-shock’ promoter. This
activates the genes to which it is attached when the scientists expose it
to heat. The scientists then introduced the original gene plus its new promoter,
into a jumping gene known as the P-element extracted from Drosophila. This
P-element cuts into the genome of the fly to insert the new construction.
The Canadian team used this P-element to inject the construction into the
cells of the flies when they were still embryos, using tiny needles.
The scientists found that the enzyme’s gene can attach itself randomly
onto various locations in the chromosomes of the flies, but on average the
flies showed a 20-fold increase over a control batch of flies in their resistance
to insecticide. Wild and Phillips believe that the difference in the resistance
to insecticide between the tissue culture and the flies is due to a protein
which fixes the enzyme to the fly’s cell walls. This prevents the enzyme
from moving to a site within the insect where it might be most effective.
The team now aims to remove this anchor.
Wild believes that his work and that of the Canadians provide insights
into the mechanisms that insects use to develop a natural resistance to
insecticides, perhaps by using a similar technique. ‘They cannot be using
normal evolutionary processes to accomplish insecticide resistance because
the timetable is too short. I believe that the ability to mobilise genes
from one biological system to another is part of the genetic function .
. . there are tremendous genetic dynamics taking place. What we have presumed
to be barriers between species are not genetic barriers but only reproductive
barriers.’
Wild does not suggest that biological agents or enzyme systems are immediate
panaceas for clearing the environment of pesticides. Enzymes such as these
need fairly mild conditions to function efficiently. Their surroundings
must not be too acidic or too hot, and the enzymes need water to function
well. Wild is not ready to test his modified fruit flies in the environment,
so he has not applied for a licence to do so.
Wild says that he has yet to convince himself, and his peers in the
scientific community, that release of the added gene will cause no genetic
damage. It could be dangerous to release such flies without restricting
the activity of their new gene to ensure that their resistance could not
spread to other insects and make today’s pesticides worthless.
