Surface signaling in ferric citrate transport gene induction: Interaction of the FecA, FecR, and FecI regulatory proteins

In Escherichia coli, transcription of the ferric citrate transport genes fecABCDE is controlled by a novel signal transduction mechanism that starts at the cell surface. Binding of ferric citrate to the outer membrane protein FecA initiates a signal that is transmitted by FecR across the cytoplasmic membrane into the cytoplasm where FecI, the sigma factor, is activated. Interaction between the signaling proteins was demonstrated by utilizing two methods. In in vitro binding assays, FecR that was His tagged at the N terminus [(His)(10)-FecR] and bound to a Ni-nitrilotriacetic acid agarose column was able to retain FecA, and FecR that was His tagged at the C terminus [FecR-(His)(6)] retained FecI on the column. An N-terminally truncated, induction-negative but transport-active FecA protein did not bind to (His)(10)-FecR. The in vivo assay involved the determination of the FecA, FecR, and FecI interacting domains with the bacterial two-hybrid Lex-based system, FecA, interacts with FecR(101-317) and FecR(1-85) interacts with FecI(1-173). These data clearly support a model that proposes interaction of the periplasmic N terminus of FecA with the periplasmic C-terminal portion, of FecR and interaction of the cytoplasmic N terminus of FecR with FecI, which results in FecI activation.