Bacillus thuringiensis entomocidal protoxin gene sequence and gene product analysis

Abstract

A 3778-bp DNA sequence of the insecticidal protoxin gene coding sequence and flanking regions from Bacillus thuringiensis subspecies berliner 1715 has been determined. The protoxin is composed of 1155 amino acids, deduced from the nucleotide sequence, and has a calculated molecular mass of 130,615 daltons. To determine the DNA portion that encodes toxicity, sequential deletions were constructed from the 3' end of the coding region using nuclease Bal-31. Using these mutants in an insect bioassay, we found that an amino-terminal 612-amino-acid peptide is toxic, whereas, a 603-amino-acid peptide is not toxic to insects. Ninety percent of the amino acid residues were homologous to the protoxins from closely related subspecies kurstaki HD-1-Dipel and sotto. The differences occurred both in the amino-terminal half, or toxic portion, and in the carboxy-terminal half. These differences were clustered in several regions. From comparative analysis of subspecies berliner and kurstaki, we propose a model whereby the protoxin molecule is divided into distinct structural and functional domains. These domains may be responsible for the differences in specific toxicities and spectra of insect host range among these subspecies.