Bacillus thuringiensis Cry and Cyt mutants useful to counter toxin action in specific environments and to overcome insect resistance in the field

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Cry and Cyt toxins produced by Bacillus thuringiensis are insecticidal proteins used worldwide to control different insect pests, either as spray products or expressed in transgenic Bt-crops. Extensive studies have shown that they are safe to the environment and non-toxic to other organisms. However, still there are concerns regarding to the potential impact of Bt-crops on non-target organisms. One possibility to counteract the action of Cry or Cyt toxins is the use of dominant negative mutants also called anti-toxin that could inhibit the toxicity of their corresponding native toxin. Additionally, the evolution of insect resistance threatens the effectiveness of Bt-crops, since several examples of insects resistant to Bt-crops have been recently documented. The study of the mode of action of Cry and Cyt toxins has shown that oligomerization is an important step to form pores in the target midgut cells to eventually kill the larvae. This knowledge allowed us to develop strategies to produce toxin mutants that act as anti-toxins to counter Cry or Cyt toxins action in specific environments and to construct CryMod toxins that are able to kill insects that developed resistance to native Cry toxins.We will review the constructions of Cry and Cyt anti-toxins. The non-toxic helix α-4 mutants of Cry1Ab are able to oligomerize and to interact with native toxin forming inactive hetero-oligomers, inhibiting toxicity of native Cry1Ab. On the other hand, the N-terminal domain of Cyt1Aa induces oligomerization of native toxin, and inhibits its toxicity against mosquitoes as well as its hemolytic activity.Regarding insect resistance, we will review the construction of Cry1AMod toxins lacking helix α-1 that are able to form oligomers in absence of binding to cadherin receptor. CryMod toxins kill resistant insects to native Bt toxins affected in cadherin. We will review and discuss recent data that indicated that CryMod toxins overcome other mechanisms of resistance.Overall the mutants reviewed here support that oligomerization is an important step in the mechanism of action of Cry and Cyt toxins.