Inhibition of Bacillus thuringiensis proteases and their effects on crystal toxin proteins and cell-free translations

Abstract

Proteases produced during growth and sporulation of four strains of Bacillus thuringiensis were examined. Low levels of proteolytic activity were detected during the late exponential phase of growth in all four strains: two strains of B. thuringiensis subsp. kurstaki and one strain each of subsp. israelensis and berliner. In all strains, protease activities increased dramatically at the onset of sporulation. The principal proteases, both extra- and intra-cellular, were neutral, metal-loproteases. The pH optima and substrate specificities of proteases extracted from cells in various stages of growth and sporulation indicated that substantial diversity existed among the strains. Intracellular proteases from all four strains converted the 135 000 molecular weight protoxin of strain HD251, an isolate previously shown to have reduced intracellular proteolytic activity and which normally does not contain a protein of the lower molecular weight, to a 68 000 molecular weight toxin. Cell-free translational activity of extracts from strain HD251 were approximately fivefold more active than were equivalent extracts from strain LB1 presumably because of the reduced intracellular proteolytic activity of this strain.