In vitro-binding of the natural siderophore enantiomers pyochelin and enantiopyochelin to their AraC-type regulators PchR in Pseudomonas.

The enantiomeric siderophores pyochelin and enantiopyochelin of Pseudomonas aeruginosa and Pseudomonas protegens promote growth under iron limitation and activate transcription of their biosynthesis and uptake genes via the AraC-type regulator PchR. Here we investigated siderophore binding to PchR in vitro using fluorescence spectroscopy. A fusion of the N-terminal domain of P. aeruginosa PchR with maltose binding protein (MBP-PchR'PAO) bound iron-loaded (ferri-) pyochelin with an affinity (Kd) of 41 ± 5 μM. By contrast, no binding occurred with ferri-enantiopyochelin. Stereospecificity of a similar fusion protein of the P. protegens PchR (MBP-PchR'CHA0) was less pronounced. The Kd's of MBP-PchR'CHA0 for ferri-enantiopyochelin and ferri-pyochelin were 24 ± 5 and 40 ± 7 μM, respectively. None of the proteins interacted with the iron-free siderophore enantiomers, suggesting that transcriptional activation by PchR occurs only when the respective siderophore actively procures iron to the cell.