A novel expression vector based on the rmf promoter (Wada et al. Proc. Natl. Acad. Sci. USA (1995) 87: 2657-2661) was developed. Using lacZ gene as a model, protein production depending on cell growth phase and growth rate was found. In addition, by subjecting the recombinant cells to temperature downshift and substrate famine, the strain was capable of producing 35 000-40 000 units of beta -galactosidase. These characteristics indicate the potential application of this expression vector to bioprocessing such as protein production in continuous or batch fermentation, biotransformation using entrapped cells, and in situ bioremediation.