Structural Basis of a Novel Heme Binding Bacterial One-Component Switch

One-component systems (OCSs) integrate sensory and effector functions within a single protein, enabling rapid gene expression changes in response to environmental cues. Here, we characterized a novel putative OCS protein, FG214, from Fimbriimonas ginsengisoli, which drew our attention as a potential redox or O₂-regulated helix-turn-helix (HTH)-Per-ARNT-Sim (PAS) transcription factor. Data supporting this included our observation of the FG214 PAS domain binding a hexacoordinate heme b in oxidized conditions and undergoing a slate of redox and ligand-dependent conformational changes, transitioning from a monomer to a homodimer. Spectroscopic and structural data revealed that oxidation stabilizes the likely HTH-PAS intramolecular domain interface, while reduction of the heme iron dissociates the HTH, freeing previously-sequestered homodimerization surfaces. Similar effects were seen by addition of a small molecule ferric heme ligand, as directly visualized with a 1.47 Å crystal structure of an imidazole-bound truncated construct. Using in vitro DNA-binding assays, we identified an artificial promoter sequence and demonstrated ligand-enhanced protein-DNA binding. Finally, we performed proof of concept experiments exploring the ability of FG214 to homodimerize in vivo, setting the stage for a redox or gas sensitive biosensor. Together, these findings define FG214 as a novel heme-binding PAS DNA binding protein and potential transcription factor, complementing known heme-PAS two-component signaling switches.