Polymorphic study of chitinase catalytic domains from Bacillus thuringiensis isolates

Abstract

The coding regions of the chitinase catalytic domains (CDs) from eight Bacillus thuringiensis isolates from Tunisian soil were cloned and sequenced. The cloned sequences consist of 255 nucleotides encoding amino acid fragments of 85 residues. Nucleotide and amino acid sequences analysis showed the novelty of two of them and the identity of six to that of Chi255 chitinase from B. thuringiensis subsp. kurstaki BUPM255 taken as reference. In fact, the two CDs coding regions CDchiA85 and CDchiA173, obtained from isolates BUPM85 and BUPM173, respectively, present the highest identity of 99% to 26 published chitinases from B. thuringiensis and B. cereus. Indeed, two nucleotide transversions of G to T and of A to C, occurring at positions 156 and 109 of CDchiA85 and CDchiA173, respectively caused substitutions at the amino acid sequence level. They are mutation E192D in the CD of chitinase ChiA85 and mutation F177V in that of ChiA173. The occurrence of changes at these positions generates polymorphisms that, obviously, do not occur at the highly conserved amino acids reported to be essential for chitin hydrolysis. Insights on the correlation of this polymorphism with the corresponding chitinases were brought by means of molecular modelling. We supposed that while the polymorphism E192D could have indirect effect on the catalytic activity of the corresponding chitinase, the residue valine brought by the polymorphism F177V could interact with the aromatic residue W210 located between the most important catalytic amino acids (D209&E211); thereby suggesting its effect on catalysis. Therefore, the study taken into account the overall folding of the subject chitinases is intended for further work to ensure the relationship.