A 20-kilodalton protein preserves cell viability and promotes CytA crystal formation during sporulation in Bacillus thuringiensis.

The effect of a 20-kDa protein on cell viability and CytA crystal production in its natural host, Bacillus thuringiensis, was studied by expressing the cytA gene in the absence or presence of this protein. In the absence of the 20-kDa protein, B. thuringiensis cells either were killed during sporulation (strain cryB) or produced very small CytA crystals (strain 4Q7). Expression of cytA in the presence of the 20-kDa protein, however, preserved cell viability, especially in strain cryB, and in both strains yielded bipyramidal crystals of the CytA protein that were larger than those of wild-type B. thuringiensis. These results suggest that the 20-kDa protein promotes crystal formation, perhaps by chaperoning CytA molecules during synthesis and crystallization, concomitantly preventing the CytA protein from interacting lethally with the bacterial host cell.