Purification of the Insecticidal Toxin in Crystals of Bacillus thuringiensis

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Abstract
Crystals were purified from 4 serotypes of the insect pathogen B. thuringiensis. Crystals from these serotypes were similar in amino acid and N-terminal analyses, but differed in their toxicity to 2 spp. of Lepidoptera and in their immunological properties. Toxic polypeptides were obtained following trypsin digestion of solutions of the crystals. In 2 strains (serotypes 3 and 9), this fraction contained only 1 polypeptide. Similar results were obtained when dissolved crystals were digested with other proteolytic enzymes or with gut contents from Pieris brassicae. The trypsin-resistant polypeptide was further purified by gel and ion-exchange chromatography and had a MW of about 70,000, estimated by gel chromatography and gel electrophoresis. No evidence was obtained for a toxin of lower MW. This purified toxin accounted for most, if not all, of the toxic activity originally present in the crystal solution and was active by injection and ingestion. The purified toxic fraction from serotype 1 appeared to contain 2 polypeptides, one of which corresponded to that found with serotypes 3 and 9. There were no major differences in the composition of crystals from different serotypes of B. thuringiensis and it is concluded that the trypsin-resistant polypeptide represents the active insecticidal toxin of the crystal.