A novel tightly regulated gene expression system was developed for Escherichia coli by applying the regulatory elements of the Pseudomonas putida F1 cym and cmt operons to control target gene expression at the transcriptional level by using p-isopropylbenzoate (cumate) as an inducer. This novel expression system, referred to as the cumate gene switch, includes a specific expression vector, pNEW, that contains a partial T5 phage promoter combined with the Pseudomonas-based synthetic operator and the cymR repressor protein-encoding gene designed to express constitutively in the host strain. The induction of transcription relies on the addition of the exogenous inducer (cumate), which is nontoxic to the culture, water soluble, and inexpensive. The characteristics and potential of the expression system were determined. Using flow cytometry and fed-batch fermentations, we have shown that, with the newly developed cumate-regulated system, (i) higher recombinant product yields can be obtained than with the pET (isopropyl-␤-D-thiogalactopyranoside [IPTG])-induced expression system, (ii) expression is tightly regulated, (iii) addition of cumate quickly results in a fully induced and homogenous protein-expressing population in contrast to the bimodal expression profile of an IPTGinduced population, (iv) expression can be modulated by varying the cumate concentration, and (v) the cumate-induced population remains induced and fully expressing even at 8 h following induction, resulting in high yields of the target protein Furthermore, the cumate gene switch described in this article is applicable to a wide range of E. coli strains.A variety of gene expression systems exist for the production of recombinant proteins and as a tool in metabolic engineering. These systems differ in the hosts, plasmids, and promoters being utilized. The variety of existing expression systems reflects the diversity, complexity, and toxicity of the proteins being produced, requiring in certain instances that the product be expressed at various concentrations (14) or at different phases of cell growth (14) and be soluble (4), secreted (13) or compartmentalized (40,45). In an attempt to meet these challenges, the search for or the development of new hosts and expression vectors is ongoing.Gene transcription can be enhanced by replacing a promoter sequence naturally associated with that gene with a sequence of a stronger promoter (17, 42). The nature of the bioprocess will dictate the promoter of choice. A process that requires high-level expression will necessitate a strong constitutive or regulated promoter. A process requiring the expression of a product with toxicity issues for the cell will benefit from a regulated expression system that does not possess basal expression under repressed conditions (26, 37). The promoters P lac , P trp , P tac , P L , P T7 , P BAD , and P lacUV5 are commonly utilized for the construction of expression vectors (1, 18). Among these, P lacUV5 , P tac , and the combined system of P T7 with P lacUV5 are widely used, be...