Abstract
Bacillus thuringiensis var. kurstaki HD-255 was found to produce an extracellular, thermostable protease after the end of the vegetative growth phase. The purified enzyme had a molecular weight of 34, 000 according to sodium dodecyl sultate-polv acrylamide gel electrophoresis and an isoelectric point of 9.0.
Its proteolytic activity was inhibited by an active-site inhibitor of serine protease, phenylmethyl-sulfonyl fluoride, and also by an SH-modifying reagent, p-chloromercuribenzoic acid, suggesting that the enzyme is one of a subfamily of SH-containing serine proteases.
The enzyme showed maximal proteolytic activity at 70°C and pH 8.5-9. The most interesting characteristic was its thermostability; it retained 88.4% of its initial activity at pH 8.7 and 60°C after more than 7hr incubation in the presence of 2mM CaCl2.
