A series of new expression vectors (pPro) have been constructed for the regulated expression of genes in Escherichia coli. The pPro vectors contain the prpBCDE promoter (P(prpB)) responsible for expression of the propionate catabolic genes (prpBCDE) and prpR encoding the positive regulator of this promoter. The efficiency and regulatory properties of the prpR-P(prpB) system were measured by placing the gene encoding the green fluorescent protein (gfp) under the control of the inducible P(prpB) of E. coli. This system provides homogenous expression in individual cells, highly regulatable expression over a wide range of propionate concentrations, and strong expression (maximal 1,500-fold induction) at high propionate concentrations. Since the prpBCDE promoter has CAP-dependent activation, the prpR-P(prpB) system exhibited negligible basal expression by addition of glucose to the medium.