[Domain swapping of Cry1Aa and Cry1Ca from Bacillus thuringiensis influence crystal formation and toxicity]

Bacillus thuringiensis has been successfully used for agricultural pest and medical insect control with its significant benefits based on environmental and safety considerations. However, the deficiency of this pesticide, such as limited spectrum of insecticidal activity, low toxicity to the targets and the inducement of insect resistance, results in the urgent need to exploit new resources of B. thuringiensis or to modify known strains by genetic engineering. In this study, six recombinant Bacillus thuringiensis BT-ACC, BT-AAC, BT-ACA, BT-CAA, BT-CCA and BT-CAC were constructed through the domain swapping between crystal protein CrylAa and CrylCa. SDS-PAGE and Western blot revealed that only the recombinant BT-CAA and BT-CCA produced a 135kDa chimeric protein CrylCAA and CrylCCA respectively, but the production level was lower than the native protein CrylAa and CrylCa. These chimeric crystal proteins could be activated by trypsin, giving a 65kDa protease-resistant core toxin as the native crystal proteins CrylAa and CrylCa. Electron microscopy study indicated that the two chimeric proteins could be produced and accumulated as spherical or granules crystals during sporulation of BT-CAA and BT-CCA, whereas native CrylAa and CrylCa were accumulated as bipyramidal crystals. Unexpectedly, the toxicity of purified chimeric crystal proteins Cryl CAA and Cryl CCA was 3 - 5 times lower to Spodoptera exigua, and 190 - 260 times lower to Helicoverpa armigera than that of native CrylAa and CrylCa. These data suggested that the domain swapping of different crystal proteins might influence the formation and toxicity to targets of chemeric crystal proteins.