Secretory expression in Escherichia coli and single-step purification of a heat-stable alkaline protease

Bacteriocin release proteins (BRPs) can be used for the release of heterologous proteins from the Escherichia coli cytoplasm into the culture medium. The gene for a highly thermostable alkaline protease was cloned from Bacillus stearothermophilus F1 by the polymerase chain reaction. The recombinant F1 protease was efficiently excreted into the culture medium using E. coli XL1-Blue harboring two vectors: pTrcHis bearing the protease gene and pJL3 containing the BRPs. Both vectors contain the E. coli lac promoter–operator system. In the presence of 40μM IPTG, the recombinant F1 protease and the BRP were expressed and mature F1 protease was released into the culture medium. This opens the way for the large-scale production of this protease in E. coli. The recombinant enzyme was purified through a one-step heat treatment at 70 °C for 3 h and this method purified the protease to near homogeneity. The purified enzyme showed a pH optimum of 9.0, temperature optimum of 80 °C, and was stable at 70 °C for 24 h in the pH range from 8.0 to 10.0. The enzyme exhibited a high degree of thermostability with a half-life of 4 h at 85 °C, 25 min at 90 °C, and was inhibited by the serine protease inhibitor phenylmethylsulfonyl fluoride (PMSF).