REGULATION OF CITRATE-DEPENDENT IRON TRANSPORT OF ESCHERICHIA-COLI - FECR IS REQUIRED FOR TRANSCRIPTION ACTIVATION BY FECI

Citrate-dependent Fe3+ transport into Escherichia coli K-12 is induced by iron and citrate. The inducer is probably ferric dicitrate which does not have to be taken up into the cytoplasm to induce transcription of the fec transport genes. Two regulatory genes, fecl and fecR, located upstream of the fecABCDE transport genes, are required for induction. We report that in vivo the chromosomally encoded Fecl protein activates transcription of the fecA and fecB transport genes in response to ferric citrate and the FecR protein. Cells expressing chromosomally and plasmid-encoded truncated FecR derivatives no longer responded to ferric citrate and expressed the fec transport genes constitutively. The smallest active FecR derivative contained 59 amino acid residues as compared to the 317 residues of wild-type FecR. Constitutive induction was lower than induction of the FecR wild-type strain by ferric citrate. It is concluded that the N-terminal portion of FecR activates Fed and that the C-terminal portion of FecR responds to ferric citrate. Transcription of the fec transport genes is positively regulated by Fed and FecR and negatively regulated by the Fe2+-Fur repressor. Transcription activation and repression may occur independently of each other.