Development and Application of Whole-Cell Biosensors for the Detection of Gallic Acid.

Gallic acid is a prevalent secondary plant metabolite distinguished as one of the most effective free-radical scavengers among phenolic acids. This compound is also known for its cytotoxic, anti-inflammatory, and antimicrobial activities. Bulk quantities of gallic acid are conventionally produced by acid hydrolysis of tannins, a costly and environmentally hazardous process. With the aim to develop more sustainable approaches, microbial bioproduction strategies have been attempted recently. To advance synthetic biology and metabolic engineering of microorganisms for gallic acid production, we characterize here a transcription factor-based inducible system GalR/P that responds to the extracellular gallic acid in a dose-dependent manner in KT2440. Surprisingly, this compound does not mediate induction when GalR/P is used in non-native host background. We show that the activation of the inducible system requires gallate dioxygenase activity encoded by gene. The 4-oxalomesaconic acid, an intermediate of gallic acid-metabolism, is identified as the effector molecule that interacts with the transcription factor GalR mediating activation of gene expression. Introduction of gene along enables development of biosensors suitable for detection and monitoring of gallic acid extracellularly using non-native hosts such as and . Moreover, the -based biosensor's applicability is demonstrated by detecting and measuring gallic acid in extracts of leaves. This study reports the strategy, which can be applied for developing gallic acid biosensors using bacterial species outside genus.