Expression of pertussis toxin subunit S4 as an intracytoplasmic protein in Bacillus subtilis

The expression and secretion of pertussis toxin subunits S1 to S5 in Bacillus subtilis by the aid of a bacillar signal sequence has been reported. While secretion of subunit S1 was high, that of others was low. Ways have now been explored to improve the yield, using S4 as an example. The addition of a protease inhibitor was found to increase the amount of S4 in the culture supernatant, but the final amount was still much below that of S1. However, intracellular expression of S4 gave a high yield (500 mg I⁻¹) and the aggregated protein could easily be isolated in a few simple steps.