The mutation R423S in the Bacillus thuringiensis hybrid toxin CryAAC slightly increases toxicity for Mamestra brassicae L.

Bacillus thuringiensis Cry1Ac toxin is 100 times less toxic than Cry1C to Mamestra brassicae. An R423S mutation abolishes Cry1Ac toxin proteolysis in M. brassicae gut juice but does not increase its toxicity to this insect. The CryAAC hybrid toxin (1Ac/1Ac/1Ca) is toxic to M. brassicae but is susceptible to gut protease digestion at the R423 residue. Accordingly we have investigated the effect of the R423S mutation in CryAAC on its toxicity for M. brassicae and Pieris brassicae. Bioassays demonstrated that the R423S mutation slightly increased the toxicity of CryAAC for M. brassicae by having a significantly inhibitory effect on the growth of surviving larvae. The mutant hybrid was still highly toxic to P. brassicae. Features of CryAACR423S such as, (1) stability in M. brassicae gut juice and (2) crystal solubility were investigated. Computer simulations suggest that a possible major increase in flexibility in the CryAAC loop β7/β8 (G391–P397) caused by the R423S substitution could be a reason for the increase in M. brassicae toxicity.