Snapshot Issue 14 September 2005

The cry proteins

Though ladybirds and butterfly may look good on your garden roses, their larvae are not welcome on various agricultural crops that they infest and damage. One way to cope with this is to introduce the larvae to an insecticidal bacterium, Bacillus thuringiensis, that they will swallow and regret. Bacillus thuringiensis, or Bt, has been used for decades for just this reason by farmers worldwide.

There are different varieties of Bt which kill off different kinds of insects: butterflies, moths, flies, beetles and mosquitoes to name but a few. Bt was discovered at the very beginning of the 20th century by a Japanese biologist, Shigetane Ishiwatari, who was investigating into the cause of the killing of large populations of silkworm. It took a further decade for the German scientist Ernst Berliner to baptize it Bacillus thuringiensis, named after the German town Thuringia, where it had killed a Mediterranean flour moth and from which it had been isolated.

The pesticidal properties of Bt are due to the effects of a protein – an endotoxin – which punctures the insect’s midgut, thus causing it to leak uncontrollably and consequently bringing about death. Surprisingly, endotoxins are stored in the form of protein crystals – unusual in the animate world – in the bacteria just before sporulation and, as a result, are frequently referred to as ‘cry proteins’. Different types of endotoxins produce different types of protein crystals, and scientists are now capable of discerning a type of pesticide from the shape of the crystals.

Once the crystals have been ingested by the larvae, they are solubilized and cleaved by a gut protease to produce the active toxin: a delta-endotoxin. The latter bore a hole in the insect’s midgut epithelium thus causing chaotic ion transfer and bringing the insect’s pH to a level in which the bacteria can flourish and cause septicemia.

Currently, almost one hundred different Bt endotoxin crystal proteins are known. One Bt can carry more than one type of endotoxin and thus be effective against a number of different insects. The endotoxin genes are found in the bacteria’s plasmids, and since plasmids can migrate from one bacterium to another, this creates a natural combination of toxins and confers novel insecticidal properties to bacteria.

Bacillus thuringiensis has been used commercially – in the form of dried spores and crystal toxins – in agriculture since the 1930s. Its use increased in the 1980s when it was clear that insects were becoming resistant to synthetic insecticides which were harmful to the environment anyway. Bt is organic and affects only specific insects. With biotechnology flourishing, it was not long before specific Bt endotoxin genes were integrated into plant genomes and genetically modified crops were created: the ‘Bt crops’ which now include ‘Bt corn’, ‘Bt potato’, ‘Bt cotton’ and ‘Bt soybean’.

The advantage here is that farmers do not have to spray crops with Bt and that only the insects which harm the crops are attacked; even those which have a go at the plants’ roots. No harm can come to humans or other mammals either, since not only are toxins ineffective in mammalian physiological pH but they do not have receptors for the toxins anyway. The drawback though is that target insects are constantly exposed to toxins and, with time, they find a way to develop resistance to them...

Pesticidal crystal protein cry1Aa, Bacillus thuringiensis subsp. kurstaki: P0A366
Pesticidal crystal protein cry1Ab, Bacillus thuringiensis subsp. kurstaki: P0A370
Pesticidal crystal protein cry1Ac, Bacillus thuringiensis subsp. kurstaki: P05068

L'édition française de cette chronique est disponible dans l'Instantanés du mois de Prolune.

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