The parent Bt11 line of maize was developed to resist European Corn
Borer (Ostrinia nubilalis) by producing its own insecticide. This
event was genetically engineered through introduction of the cry1Ab
gene, isolated from the common soil bacterium Bacillus
thuringiensis (Bt). The cry1Ab gene produces the insect control
protein Cry1Ab, a delta-endotoxin. The Cry1Ab protein expressed in
Bt11 is identical to that found in nature and in commercial Bt
spray formulations. Cry proteins, of which Cry1Ab is only one, act
by selectively binding to specific sites localized on the lining of
the midgut of susceptible insect species. Following binding, pores
are formed that disrupt midgut ion flow, causing gut paralysis and
eventual death from bacterial sepsis. Cry1Ab is lethal only when
eaten by the larvae of lepidopteran insects (moths and
butterflies), and its specificity of action is directly
attributable to the presence of specific binding sites in the
target insects. There are no binding sites for delta-endotoxins of
B. thuringiensis on the surface of mammalian intestinal cells,
therefore, livestock animals and humans are not susceptible to
these proteins.
In addition to insect resistance, Bt11 was developed to allow for
the use of glufosinate ammonium, the active ingredient in
phosphinothricin herbicides (Basta®, Rely®, Liberty®, and Finale®)
as a weed control option, and as a breeding tool for selecting
plants containing the cry1Ab gene. Bt11 contains the pat gene,
isolated from a common soil actinomycete Streptomyces
viridochromogenes. This gene allows for the production of the
enzyme phosphinothricin N-acetyltransferase (PAT) which confers
tolerance to glufosinate. The PAT enzyme in maize line Bt11
converts L-phosphinothricin (PPT), the active ingredient in
glufosinate ammonium, to an inactive form. In the absence of PAT,
application of glufosinate leads to reduced production of the amino
acid glutamine and increased ammonia levels in the plant tissues,
resulting in the death of the plant. The PAT enzyme is not known to
have any toxic properties.
The parent GA21 line of maize was genetically engineered, by
particle acceleration (biolistic) transformation, to be tolerant of
glyphosate-containing herbicides. The isolated endogenous maize
epsps gene was modified through site-directed mutagenesis, such
that its encoded enzyme was insensitive to inactivation by
glyphosate, and inserted into the inbred AT maize variety. The
modified maize line permits farmers to use glyphosate-containing
herbicides for weed control in the cultivation of maize.
Glyphosate specifically binds to and inactivates the enzyme
5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), which is part
of an important plant biochemical pathway called the shikimate
pathway. The shikimate pathway is involved in the biosynthesis of
the aromatic amino acids tyrosine, phenylalanine and tryptophan, as
well as other aromatic compounds. When conventional plants are
treated with glyphosate they cannot produce the aromatic amino
acids essential to their survival. The EPSPS enzyme is present in
all plants, bacteria and fungi, but not in animals, which do not
synthesize their own aromatic amino acids. Thus, EPSPS is normally
present in foods derived from plant and microbial sources.
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