(A) The Nisin-Induced Expression System (NICE), which consists of the sensor protein NisK, the response regulator NisR, and the inducible promoter PnisA. (B) The Xylose-Induced Expression System (XIES), which consists of xylose, XylR, glucose, and P_xylT as the inducer, response-regulator, inhibitor, and inducible promoter, respectively. (C) The pZn system, which is based on zinc (Zn) starvation, the ZitR response-regulator, and the P_Zn promoter. (D) The P170 system, based on the P170 promoter, which is stimulated by an acidic (pH < 6.0) environment. (E) The Stress-Induced expression System (SICE). Stresses including acidic pH, salt stress, and heat shock stimulate the GroESL promoter. Stimulation of the promoter drives expression of the target protein. Created with BioRender.com.