[Differences in primary structures of delta-endotoxins produced by various serotypes of Bacillus thuringiensis]

The primary structures of delta-endotoxins (crystal-forming proteins) produced by two serotypes of Bacillus thuringiensis--V (var. galleriae) and III (var. alesti) were compared. These proteins differ by the specificity of their action on Lepidoptera larvae as well as by their molecular weights. To evaluate the homology of primary structures the tryptic hydrolysates of both endotoxins were fractionated by ion-exchange and thin-layer cellulose chromatography with a subsequent amino acid determination in the hydrolysates of thus obtained the fraction. Only 10%, of the fractions gained from the tryptic hydrolysates of the two delta-endotoxins had similar amino acid composition. In control experiments on comparison of two hydrolysates of the same delta-endotoxins the percentage of fractions with similar amino acid composition was as high as 50%, which reflects the tendency of this approach to overestimate the extent of the differences between the two sequences. Hence the delta-endotoxins produced by two serotypes of Bacillus thuringiensis, being clearly homologous, reveal substantial differences in their amino acid sequences, which are dispersed along the whole polypeptide chain. These striking differences in the primary structures are indicative of an unusually high rate of their evolution, which may be of functional importance for B. thuringiensis serotypes adaptation to different ecological niches.