This domain is found at the C-terminal of FecR [1,2], which adopts a secretin N0-like fold [1]. FecARI are responsible for induction of the ferric citrate uptake system. When ferric citrate binds FecA, a signal is transmitted across the outer membrane to FecR, a cytoplasmic membrane protein that transmits signal across the membrane and activates the sigma-70 family protein FecI [2].

1: Structural basis of cell-surface signaling by a conserved sigma regulator in Gram-negative bacteria. Jensen JL, Jernberg BD, Sinha SC, Colbert CL; J Biol Chem. 2020;295:5795-5806. PMID:32107313

2: Surface signaling in ferric citrate transport gene induction: interaction of the FecA, FecR, and FecI regulatory proteins. Enz S, Mahren S, Stroeher UH, Braun V; J Bacteriol. 2000;182:637-646. PMID:10633096