The combinatory effect of Cyt1Aa flexibility and specificity against dipteran larvae improves the toxicity of Bacillus thuringensis kurstaki toxins

Abstract

Cyt1A98 is a novel cytolytic protein, from BUPM98 Bacillus thuringiensis strain, characterized by its synergistic activity with B. thuringiensis kurstaki toxins against lepidopteran larvae. In this study, we evidenced that Cyt1A98 improves the toxicity of B. thuringiensis kurstaki toxins against Aedes aegypti larvae. In fact, the strain BNS3pHTcyt1A98 exhibited a larvicidal activity of about 849-fold of that of BNS3pHTBlue against A. aegypti. The molecular and biochemical characterizations, of cyt1A98 gene and its product, were achieved. Cyt1A98 had an LC₅₀ value of about 126.56 mg l^-1 against A. aegypti larvae. Compared to Cyt1Aa of B. thuringiensis israelensis, Cyt1A98 amino acid sequence harbours three substitutions of three conserved amino acids among Cyt1Aa family members (Ser42Pro, Pro82Ala, Met188Thr). The Cyt1A98 protein structural analysis evidenced more flexibility than Cyt1Aa. According to the high fluctuation observed for the residue Pro42, the amino acid at position 42 is implicated in the flexibility property of Cyt1Aa especially for the αC and αD helices, involved in the penetration into the cell membrane. The toxicity improvement could be probably due to the higher flexibility combined with the specific affinity toward dipteran larvae. The Cyt1A/B. thuringiensis kurstaki Cry toxins model provides a potential molecular genetic strategy for an efficient bioinsecticide.

Keywords: Cyt1A98 expression; Cyt1Aa flexibility; Larvicidal activity improvement.