Parasporal Body of Bacillus thuringiensis israelensis STRUCTURE, PROTEIN COMPOSITION, AND TOXICITY

Abstract

Since its discovery inJapan and Germany during the early part of this century, more than 25 subspecies of the spore-forming insecticidal bac­ terium Bacillus thuringiensis Berliner have been described (de Barjac 1985). The most distinctive characteristic of this bacterium is a parasporal body produced during sporulation that consists primarily of insecticidal pro­ teins (Angus 1965; Heimpe11967; Aronson, Beckman, and Dunn 1986; H6fte and Whiteley 1989). In most subspecies, the parasporal body is a bipyramidal crystal containing one or more similar proteins of about 135 kDa that are tox­ ic to lepidopterous larvae. When ingested by a larva, this toxin-containing inclusion dissolves in the alkaline gut juices, and midgut proteases cleave the protoxin, yielding an active peptide toxin of 60-70 kDa, the ?)-endotoxin. Although the toxin's precise mode of action is not fully understood, intoxica­ tion results in an osmotic imbalance across the midgut epithelial cell mem­ brane, which leads quickly to hypertrophy and lysis of midgut cells. Lysis is followed by disruption of the basement membrane, leakage of digestive juices into the hemocoel, and larval death (Luthy and Ebersold 1981). The ?)­ endotoxins of different subspecies of B. thuringiensis can vary considerably in toxicity to larvae. These variations are thOUght to be due to differences in the amino acid sequence of the toxins, and are currently the subject of much interest because of the potential for increasing toxicity and host spectrum through site-directed mutagenesis. The first isolate of B. thuringiensis shown to possess characteristics markedly different from those described above was ONR-60A. This isolate was collected by Goldberg and Margalit (1977) from a mosquito larval hab­ itat in the Negev Desert of Israel and demonstrated to be highly toxic to the