Three-dimensional structures of PAS (A) and PDC (B) domains. The structures are based on (A) the Rhizobium meliloti oxygen sensor FixL protein with its ligand heme (UniProt P10955: positions 122–251) (PDB: 1D06) , and (B) the ligand-binding domain of the Klebsiella pneumoniae sensor kinase CitA protein with its ligand citrate (UniProt P52687: positions 5–135) (PDB: 1P0Z) . For both structures, the core β strands are labeled from 1 to 5. Schematic models were generated by PyMol (Schrödinger, Portland, OR). Each region is colored as follows: the amino end with blue, the leading α-helix region with green, the first two β-strands with orange, the inter-domain α-helix region with magenta, the last three β-strands with yellow, and the carboxyl end with red. Ligands are shown in white stick models.