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ÎÄÌâÊÇ  £ºEngineering Modified Bt Toxins to Counter Insect Resistance

Õâ¸öÊÇÕªÒª£ºThe evolution of insect resistance threatens the effectiveness of Bacillus thuringiensis (Bt) toxinsthat are widely used in sprays and transgenic crops. Resistance to Bt toxins in some insects is linked
with mutations that disrupt a toxin-binding cadherin protein. We show that susceptibility to the Bttoxin Cry1Ab was reduced by cadherin gene silencing with RNA interference in Manduca sexta,confirming cadherin¡¯s role in Bt toxicity. Native Cry1A toxins required cadherin to form oligomers,
but modified Cry1A toxins lacking one a-helix did not. The modified toxins killed cadherin-silencedM. sexta and Bt-resistant Pectinophora gossypiella that had cadherin deletion mutations. Ourfindings suggest that cadherin promotes Bt toxicity by facilitating toxin oligomerization and
demonstrate that the modified Bt toxins may be useful against pests resistant to standard Bt toxins.


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http://www.namipan.com/d/f287b47 ... e2f4b4c24845c470300


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caitikuan

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The evolution of insect resistance threatens the effectiveness of Bacillus thuringiensis (Bt) toxinsthat are widely used in sprays and transgenic crops.
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Resistance to Bt toxins in some insects is linked with mutations that disrupt a toxin-binding cadherin protein.
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We show that susceptibility to the Bttoxin Cry1Ab was reduced by cadherin gene silencing with RNA interference in Manduca sexta, confirming cadherin¡¯s role in Bt toxicity.
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Native Cry1A toxins required cadherin to form oligomers, but modified Cry1A toxins lacking one a-helix did not.
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The modified toxins killed cadherin-silenced Manduca sexta and Bt-resistant Pectinophora gossypiella that had cadherin deletion mutations.
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Our findings suggest that cadherin promotes Bt toxicity by facilitating toxin oligomerization and demonstrate that the modified Bt toxins may be useful against pests resistant to standard Bt toxins.
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[ Last edited by caitikuan on 2009-1-15 at 10:31 ]
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Originally posted by xu_0501 at 2009-1-12 17:28:
×î½üºÜ棬¼¸¼þÊÂÇé´Õµ½Ò»ÆðÁË£¬ÓÐÆªÎÄÕÂÐèÒª·­Ò룬ûÓÐʱ¼ä×ö£¬²¢ÇÒ×Ô¼ºµÄÓ¢ÓïˮƽҲÓÐÏÞ¡£ÌØÔÚ´ËÇë¸ßÊÖ°ïæ·­Òë³ÉÖÐÎÄ¡£ÎÒä¯ÀÀÁ˹þ£¬³ýרҵ´Ê»ãÍ⣬²»ÊǺÜÄѵģ¬ ºÇºÇ¡£Èç¹ûÓиßÊÖÄÜ×ö£¬ÇëÏȻظöÌû×Ó£¬×îºÃÔÚ1. ...

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caitikuan

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The toxins produced by Bacillus thuringiensis kill some major insect pests such as mosquitoes and crop-eating caterpillars but are harmless to vertebrates and most other organisms.
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Transgenic corn and cotton producing Bt toxins grew on more than 32 million hectares worldwide in 2006.
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The primary threat to the long-term efficacy of Bacillus thuringiensis toxins is the evolution of resistance by pests.
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Many insects have been selected for resistance to Bt toxins in the laboratory, and two crop pests (Plutella xylostella and Trichoplusia ni) have evolved resistance to Bt sprays outside of the laboratory.
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The most widely used Bt toxins are crystal toxins in the Cry1A family, particularly Cry1Ab in transgenic Bt corn and Cry1Ac in transgenic Bt cotton, which kill caterpillars (lepidopteran larvae).
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Cry1A toxins bind to the extracellular domain of cadherin proteins that traverse the insect larval midgut membrane.
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Disruption of Bt toxin binding to midgut receptors is the most common mechanism of insect resistance.
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Mutations in the midgut cadherins that bind Cry1Ac are linked with
and probably cause resistance in at least three lepidopteran pests of cotton.
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caitikuan

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Two hypotheses proposed to explain how Cry1A toxins function are the pore-formation model and the signaling model.
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These theories share initial steps: Cry1A protoxins are ingested, solubilized in the gut, and cleaved by midgut proteases such as trypsin to yield activated 60-kD monomeric toxins that bind to cadherin with high affinity.
ÕâЩÀíÂ۵Ĺ²Í¬³õʼ²½Ö裺 Cry1A ǰ¶¾Ëص°°×µÄÉãÈ룬 ÔÚ³¦µÀÖÐÈܽ⣬¾­Ö㦵°°×øÈçÒȵ°°×øµÄÏû»¯·ÖÁѲúÉúºÍ¸ÆÕ³µ°°×¾ßÓиßÇ׺ÍÁ¦ÉúÎï»îÐÔµÄ60-kD µ¥Ìå¶¾ËØµ°°×¡£
The signaling model, derived from studies of insect cell cultures, suggests that after protease-activated monomeric toxins bind to cadherin, initiation of a magnesium-dependent signaling pathway causes cell death.
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In contrast, on the basis of results from in vitro experiments and bioassays, the pore-formation model proposes that protease activated monomers bind to cadherin to facilitate protease cleavage of the N-terminus of the toxin, including helix a-1of domain.
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This cleavage induces the assembly of oligomeric forms of the toxin, which have increased binding affinity to secondary receptors,including glycosylphosphatidylinositol-anchored proteins, aminopeptidase N, and alkaline phosphatases.
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