Cautionary notes on the use of arabinose- and rhamnose-inducible expression vectors in Pseudomonas aeruginosa

Abstract

The Pseudomonas aeruginosa virulence factor regulator (Vfr) is a cAMP-responsive transcription factor homologous to the Escherichia coli cyclic AMP receptor protein (CRP). Unlike CRP, which plays a central role in E. coli energy metabolism and catabolite repression, Vfr is primarily involved in the control of P. aeruginosa virulence factor expression. Expression of the Vfr regulon is controlled at the level of vfr transcription, Vfr translation, cAMP synthesis, and cAMP degradation. While investigating mechanisms that regulate Vfr translation, we placed vfr transcription under the control of the rhaBp rhamnose-inducible promoter system (designated P Rha ) and found that P Rha promoter activity was highly dependent upon vfr . Vfr-dependence was also observed for the araBp arabinose-inducible promoter (designated P BAD ). The observation of Vfr-dependence was not entirely unexpected. Both promoters are derived from E. coli where maximal promoter activity is dependent upon CRP. Like CRP, we found that Vfr directly binds to promoter probes derived from the P Rha and P BAD promoters in vitro. Because Vfr-cAMP activity is highly integrated into numerous global regulatory systems including c-di-GMP signaling, the Gac/Rsm system, MucA/AlgU/AlgZR signaling, and Hfq/sRNAs, potential exists for significant variability in P Rha and P BAD promoter activity in a variety of genetic backgrounds, and use of these promoter systems in P. aeruginosa should be employed with caution. SIGNIFICANCE Heterologous gene expression and complementation is a valuable and widely-utilized tool in bacterial genetics. The arabinose-inducible P araBAD (P BAD ) and rhamnose-inducible P rhaBAD (P Rha ) promoter systems are commonly used in P. aeruginosa genetics and prized for the tight control and dynamic expression ranges that can be achieved. In this study we demonstrate that the activity of both promoters is dependent upon the cAMP-dependent transcription factor Vfr. While this poses an obvious problem for use in a vfr mutant background, the issue is more pervasive considering that vfr transcription/synthesis and cAMP homeostasis are highly integrated into the cellular physiology of the organism and influenced by numerous global regulatory systems. Fortunately, the synthetic P Tac promoter is not subject to Vfr regulatory control.