E. coli ferric citrate transport system. The outer membrane protein FecA, which recognises ferric citrate from the extracellular medium, serves two functions. First, FecA transports ferric citrate into the periplasm, where Fe3+ (and possibly citrate) is further transported into the cytoplasm by the FecBCDE proteins. FecB is a periplasmic binding protein and FecCDE constitutes an ABC transporter (left part of figure, blue). Second, FecA regulates the induction of fecABCDE transport genes initiated by the binding of ferric citrate to FecA. This involves the N-domain of FecA, the inner membrane regulator FecR, and the cytoplasmic sigma factor FecI (right part of figure, pink). Both transport and induction require energy transduction from the TonB-ExbB-ExbD complex in the inner membrane. Ferric citrate-bound FecA is believed to interact with TonB via its TonB-box motif (central part of figure, yellow), while the N-domain of FecA interacts with FecR (right part of figure, pink). When the intracellular iron level is high, the transcriptional repressor Fur is bound with ferrous ion (Fe2+). The Fe2+-Fur complex represses the transcription of fecIR and fecABCDE genes. RNA pol, RNA polymerase.