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. FecA1–79 interacts with FecR101–317 and FecR1–85 interacts with FecI1–173. 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.

Transcription of the ferric citrate transport system of Escherichia coli K-12 is repressed by Fe(2+)-Fur and activated by ferric citrate. Ferric citrate does not have to enter the cytoplasm; it initiates a signal transduction mechanism by binding to the outer membrane receptor FecA. Presumably, a conformational change is transmitted in a TonB-dependent manner to the FecR protein. FecR activates FecI, and FecI activates transcription of the fecABCDE transport genes. In this communication, FecI was isolated after cloning fecI downstream of an ideal ribosome-binding site. Overexpressed FecI formed inclusion bodies which were solubilized and purified in active form using a mild detergent. FecI, in conjunction with RNA polymerase core enzyme, directed transcription from the fecA promoter in an in vitro run-off transcription assay. Furthermore, FecI retarded the electrophoretic mobility of a specific 75 bp DNA fragment located upstream of fecA. An in vivo competition experiment between the fecA promoters of wild-type and mutant strains identified the nucleotide positions 2747, 2749, 2751 and 2753, located within the 75 bp fragment, as important for FecI-induced transcription. Mobility band shift of fecA promoter DNA caused by cell lysates required growth of cells in the presence of ferric citrate and expression of FecA, FecI and FecR. These data support the previous assignment of FecI, based on sequence homologies, to a new subfamily of eubacterial RNA polymerase sigma 70 factors that respond to extra-cytoplasmic stimuli and regulate extracytoplasmic functions.

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Transcription of the ferric citrate transport genes is initiated by binding of ferric citrate to the FecA protein in the outer membrane ofEscherichia coli K-12. Bound ferric citrate does not have to be transported but initiates a signal that is transmitted by FecA across the outer membrane and by FecR across the cytoplasmic membrane into the cytoplasm, where the FecI extracytoplasmic-function (ECF) sigma factor becomes active. In this study, we isolated transcription initiation-negative missense mutants in the cytoplasmic region of FecR that were located at four sites, L13Q, W19R, W39R, and W50R, which are highly conserved in FecR-like open reading frames of thePseudomonas aeruginosa, Pseudomonas putida,Bordetella pertussis, Bordetella bronchiseptica, and Caulobacter crescentus genomes. The cytoplasmic portion of the FecR mutant proteins, FecR1–85, did not interact with wild-type FecI, in contrast to wild-type FecR1–85, which induced FecI-mediated fecB transport gene transcription. Two missense mutations in region 2.1 of FecI, S15A and H20E, partially restored induction of ferric citrate transport gene induction of thefecR mutants by ferric citrate. Region 2.1 of ς70 is thought to bind RNA polymerase core enzyme; the residual activity of mutated FecI in the absence of FecR, however, was not higher than that of wild-type FecI. In addition, missense mutations in the fecI promoter region resulted in a twofold increased transcription in fecR wild-type cells and a partial restoration of fec transport gene transcription in thefecR mutants. The mutations reduced binding of the Fe2+ Fur repressor and as a consequence enhancedfecI transcription. The data reveal properties of the FecI ECF factor distinct from those of ς70 and further support the novel transcription initiation model in which the cytoplasmic portion of FecR is important for FecI activity.

Ferric citrate induces transcription of the ferric citrate transport genes (fec) in escherichia coli by binding to the outer membrane receptor protein FecA without entering the cell. The signal elicited by ferric citrate crosses the outer membrane via TonB, ExbB, and ExbD. FecR transmits the signal across the cytoplasmic membrane and activates FecI located in the cytoplasm. FecI belongs to a subgroup of sigma factors that respond to extracytoplasmic stimuli. Chromosomal insertion and deletion mutations were generated in fecI; the resulting mutants were totally devoid of FecA production and fecB-lacZ expression. Iron starvation did not derepress fec transport gene transcription in fecI mutants. Missense point mutations were generated in the predicted helix-turn-helix motif of FecI to examine its role in transcription initiation. Replacement of glutamate by alanine (E141A) at the third position in the first helix reduced the residual activity of FecI in the absence of ferric citrate to 30% of the wild-type level, but induced fec transcription almost normally n the presence of ferric citrate. Mutant FecI(K145E) displayed 156% of the activity of wild-type FecI in the absence of ferric citrate and conferred full induction by ferric citrate. Mutant FecI(K155E), which has a mutation in the second helix, showed 9% of the wild-type activity in the presence of ferric citrate and 78% in the absence of ferric citrate. The reduced activity of FecI(K155E) was also shown in vitro by DNA binding assays with cell lysates; in gel retardation experiments FecI(K155E) reduced the electrophoretic mobility of fecA promoter-containing DNA less than did wild-type FecI. fecI is not autoregulated, as demonstrated by the lack of FecI-induced fecI-lacZ expression in vivo and by the lack of specific fecI transcription in vitro. Instead, formation of fecI mRNA requires sigma 70. We conclude that transcription of the fec transport genes is regulated by FecI, which responds to ferric citrate via FecR. fecI and fecR co-transcription is inhibited by the iron-loaded Fur repressor, which then results in a low level of transcription of the fec transport genes.