A Synthetic Cumate-Inducible Promoter for Graded and Homogenous Gene Expression in Pseudomonas aeruginosa

Inducible gene expression systems are powerful genetic tools to study bacterial physiology, probing essential and toxic gene functions, gene dosage effects, and overexpression phenotypes. For the opportunistic human pathogen Pseudomonas aeruginosa, dedicated inducible gene expression systems are scarce. In the current study, we developed a minimal synthetic 4-isopropylbenzoic acid (cumate)-inducible promoter, called P(QJ), that is tunable over several orders of magnitude. This was achieved by combining semirandomized housekeeping promoter libraries and control elements from the Pseudomonas putida strain F1 cym/cmt system with powerful fluorescence-activated cell sorting (FACS) to select functionally optimized variants. Using flow cytometry and live-cell fluorescence microscopy, we demonstrate that P(QJ) responds rapidly and homogenously to the inducer cumate in a graded manner at the single-cell level. P(QJ) and cumate are orthogonal to the frequently used isopropyl β-d-thiogalactopyranoside (IPTG)-regulated lacI(q)-P(tac) expression system. The modular design of the cumate-inducible expression cassette together with the FACS-based enrichment strategy presented here facilitates portability, thus serving as a blueprint for the development of tailored gene expression systems for a wide range of bacteria. IMPORTANCE Reverse genetics is a powerful approach to study bacterial physiology and behavior by relying on well-developed genetic tools, such as inducible promoters. For the human pathogen Pseudomonas aeruginosa, well-characterized inducible promoters are scarce. In the current work, we used a synthetic biology-based approach to develop a cumate-inducible promoter for P. aeruginosa, termed P(QJ), that shows excellent induction properties at the single-cell level. This genetic tool provides the means for qualitative and quantitative gene function studies describing P. aeruginosa’s physiology and virulence in vitro and in vivo. Because this synthetic approach to constructing species-specific inducible promoters is portable, it can serve as a blueprint for similar tailored gene expression systems in bacteria largely lacking such tools, including, for example, representatives of the human microbiota.