1 Department of Chemical Engineering and Applied Chemistry, University of Toronto, Canada.
2 Centre for Applied Synthetic Biology, Concordia University, Montreal, Canada.
3 Department of Biology, Concordia University, Montreal, Canada.
4 PROTEO, Quebec Network for Research on Protein Function, Structure, and Engineering, Canada.
5 Department of Chemistry and Biochemistry, Concordia University, Montreal, Canada.
6 The Institute of Biomedical Engineering, University of Toronto, Canada.
7 Department of Microbiology, Immunology and Infectious Diseases, University of Calgary, Canada.

Prokaryotic transcription factors (TFs) regulate gene expression in response to small molecules, thus representing promising candidates as versatile small molecule-detecting biosensors valuable for synthetic biology applications. The engineering of such biosensors requires thorough in vitro and in vivo characterization of TF ligand response as well as detailed molecular structure information. In this work, we functionally and structurally characterize the Pca regulon regulatory protein (PcaR) transcription factor belonging to the IclR transcription factor family. Here, we present in vitro functional analysis of the ligand profile of PcaR and the construction of genetic circuits for the characterization of PcaR as an in vivo biosensor in the model eukaryote Saccharomyces cerevisiae. We report the crystal structures of PcaR in the apo state and in complex with one of its ligands, succinate, which suggests the mechanism of dicarboxylic acid recognition by this transcription factor. This work contributes key structural and functional insights enabling the engineering of PcaR for dicarboxylic acid biosensors, in addition to providing more insights into the IclR family of regulators.