Bt toxin in GM plants is not the same as natural Bt toxin
The Bt toxin expressed by GM Bt plants is different from natural Bt, both in terms of its structure and its mode of action.1 Structurally, there is at least a 40% difference between the toxin in Bt176 maize (formerly commercialized in the EU, now withdrawn) and natural Bt toxin.2 The US Environmental Protection Agency, in its review of the commercialized Monsanto GM maize MON810, said it produced a “truncated” version of the protein – in other words, a much shorter form of the protein that is different from the natural form.3
Such changes in a protein can mean that it has very different environmental and health effects. First, the GM Bt toxin loses its selectivity and can kill non-target insects including beneficial predators. Second, GM Bt toxin can have unsuspected negative health impacts on people or animals that eat a crop containing it. The protein may be more toxic or allergenic than the natural form of the protein.
Even tiny changes in a protein can completely change its properties. For example, soybeans can be genetically engineered to tolerate a herbicide that would normally kill them by changing a gene that gives rise to a protein differing from the natural protein by just two amino acids.4 As researchers at the Centre for Integrated Research in Biosafety in New Zealand pointed out in a submission to the Australia/New Zealand GMO regulator FSANZ on the regulatory assessment of this soybean,5 a change even of a single amino acid can radically change the properties of proteins, which in turn can result in changed behaviour of a plant.6,7
In some cases, not even an amino acid change is necessary to alter the characteristics of a protein. Differences in the sequence of the DNA base units in a gene can change the properties of the resulting protein without altering the amino acid sequence.8Changes in the three-dimensional shape of the protein alone can turn harmless proteins into toxins,9,10 as demonstrated by the prion protein causing the “mad cow disease” BSE.11
Natural Bt toxin also has a very different mode of action from the Bt toxin produced in GM plants. Natural Bt is not a toxin but a protoxin. That means it only becomes toxic when subjected to certain conditions, such as when made into a solution and broken down by enzymes in the gut of the insect that eats it.
In the environment, natural Bt breaks down rapidly in daylight soon after it is sprayed, so it is unlikely to find its way into animals or people that eat the crop. With GM Bt crops, in contrast, the Bt toxin is present in every cell of the plant in pre-activated form.1,12 The plant itself becomes a pesticide, and people and animals who eat the plant are eating a pesticide.