Expression, purification and characterization of the Cry2Aa14 toxin from Bacillus thuringiensis subsp. kenyae

An indigenous strain HD-550 of Bacillus thuringiensis subsp. kenyae was found to be toxic to lepidopteran as well as dipteran insects. The cry2Aa gene (classified as cry2Aa14) from this isolate was cloned and expressed in Escherichia coli. Only a little amount of the expressed Cry2Aa14 protein was observed in soluble fraction under normal induction condition. The inclusions were non-toxic to test insects, whereas solubilized Cry2Aa14 was highly toxic to lepidopteran and dipteran insects. Cry2Aa14 protein was expressed as thioredoxin (trx) fusion protein for improving the yield of active protein. An enhancement of nearly 15% was observed in the yield of active Cry2Aa14. The TrxA-Cry2Aa14 protein purified from the solubilized fraction also showed toxicity profile similar to the wild-type protein. The LC₅₀ values of Cry2Aa14 and TrxA-Cry2Aa14 protein against Spodoptera litura was 694 and 696 ng/cm², respectively, while for Culex quinquefasciatus the LC₅₀ values were 894 and 902 ng/ml, respectively. The broad spectrum toxicity of the Cry2Aa14 thus indicates that this protein could be an important component in integrated pest management. Further, the trx tag clearly led to higher yield, which facilitates protein purification for biophysical and biochemical characterization.