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, fecI and fecR, located upstream of the fecABCDE transport genes, are required for induction. We report that in vivo the chromosomally encoded FecI 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 FecI and that the C-terminal portion of FecR responds to ferric citrate. Transcription of the fec transport genes is positively regulated by FecI and FecR and negatively regulated by the Fe2(+)-Fur repressor. Transcription activation and repression may occur independently of each other.

The fec locus encodes a citrate-mediated Fe3+ transport system. Six mutations in fec were mapped by conjugation experiments at min 7 (13) of the Escherichia coli linkage map (1), which supported the biochemical distinction (3) of the citrate iron transport system from the iron transport systems mediated by enterochelin (enterobactin) and the hydroxamate siderophores. Phage P1 cotransduction offec with markers in the 7-min region (lac and proAB) failed (5, 13). Cotransduction with argF was complicated by the occurrence of two genes, argF at min 7 and argI at min 97, both of which encode the same enzyme, ornithine carbamoyltransferase. The initial localization offec at min 7 could be due to erroneous conjugation experiments in which argI at min 97 instead of argF at min 7 was transferred.fec is composed of seven contiguous genes, of whichfecI and fecR regulate transcription of the fecABCDE transport genes (2, 7, 9, 11). Sequencing of the entire fec region (7, 9, 11) revealed an ISI element, upstream of the fecIRABCDE operon (11), which is 100% identical to the sequence of ISlF at min 97.5 (10). The other ISJ elements sequenced (listed in references 9 and 10), which include ISIB and ISIC at min 6.3 and 6.5, respectively, show only 85.5 to 85.6% sequence identity with ISI upstream of fecI. To clarify the conflicting data, we hybridized DNA probes isolated from fecR (BglI-PstI fragment), fecA (PstI-PvuI fragment), and fecB (PvuI-SalI fragment) with lambda phages of the Kohara "miniset" library carrying DNA fragments of the 7-and 97-min regions ofE. coli K-12 W3110 (6). All three fec probes hybridized to phage 7G7 (662), the fecR and fecA probes hybridized to phage 9H9 (663), and there was no hybridization to phage E4D8 (660) of the 97-min region or to phages 9G4 (134), 6E6 (135), 1F6 (136), 9F1 (137), and 10A6 (138) of the 7-min region. This result clearly demonstrates that thefec genes are located at min 97.3 of the E. coli K-12 chromosome and not at min 7 (Fig.

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