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 fe cABCDE is controlled by a novel signal transduction mechanism that starts a t 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. In teraction between the signaling proteins was demonstrated by utilizing two methods. In in vitro binding assays, FecR that was His tagged at the N term inus [(His)(10)-FecR] and bound to a Ni-nitrilotriacetic acid agarose colum n 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, indu ction-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 F ecI interacting domains with the bacterial two-hybrid Lex-based system, Fec A, 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 peripla smic N terminus of FecA with the periplasmic C-terminal portion, of FecR an d interaction of the cytoplasmic N terminus of FecR with FecI, which result s in FecI activation.