Summary:

1.Transport of vitamin B12 across the cytoplasmic membrane of Escherichia coli requires the products of btuC and btuD, two genes in the btuCED operon. Despite its genetic location in the transport operon, the btuE product plays no essential role in vitamin B12 transport. (Mol Gen Genet 1989 Jun;217(2-3):301-8 Vitamin B12 transport in Escherichia coli K12 does not require the btuE gene of the btuCED operon. Rioux CR, Kadner RJ PMID: 2671656, UI: 89364713).

2.Ferric citrate induces transcription of the ferric citrate transport genes fecABCDE in Escherichia coli by binding to the outer-membrane receptor protein FecA without entering the cell. Replete iron concentrations inhibit transcription of the fec transport system via the iron-loaded Fur repressor. (Arch Microbiol 1998 Jun;169(6):483-90 Iron regulates transcription of the Escherichia coli ferric citrate transport genes directly and through the transcription initiation proteins. Angerer A, Braun V PMID: 9575233, UI: 98241334).

3.Citrate and iron have to enter only the periplasmic space in order to induce the citrate-dependent iron(III) transport system of Escherichia coli. The five transport genes fecABCDE form an operon and are transcribed from fecA to fecE. Two genes, termed fecI and fecR, that mediate induction by iron(III) dicitrate have been identified upstream of fecA. The amino acid sequence of the FecI protein contains a region close to the carboxy-terminal end for which a helix-turn-helix motif is predicted, which is typical for DNA-binding regulatory proteins. The FecI protein was found in the membrane, and the FecR protein was found in the periplasmic fraction. It is proposed that the FecR protein is the sensor that recognizes iron(III) dicitrate in the periplasm. The FecI protein activates fec gene expression by binding to the fec operator region. In the absence of citrate, FecR inactivates FecI. The lack of sequence homologies to other transmembrane signaling proteins and the location of the two proteins suggest a new type of transmembrane control mechanism. (J Bacteriol 1990 Dec;172(12):6749-58 Novel two-component transmembrane transcription control: regulation of iron dicitrate transport in Escherichia coli K-12. Van Hove B, Staudenmaier H, Braun V PMID: 2254251, UI: 91072220 ).

4.In Escherichia coli, iron assimilation by means of the siderophore enterobactin requires two hydrophobic cytoplasmic membrane proteins, FepD and FepG, which are essential components of a binding-protein-dependent transport system. (J Gen Microbiol 1992 Oct;138 ( Pt 10):2167-71 Identification of hydrophobic proteins FepD and FepG of the Escherichia coli ferrienterobactin permease. Chenault SS, Earhart CF PMID: 1479347, UI: 93123972). B0591, encodes a membrane protein that is not essential for ferric enterobactin transport. Two others, fepD and fepG, were found to be essential for transport and their translational products showed extensive homology to other integral membrane proteins involved in TonB-dependent transport processes. The FepC amino acid sequence suggested a peripheral membrane location and revealed conserved ATP-binding domains. Together these data indicate that ferric enterobactin is transported through a typical periplasmic binding protein-dependent system. In addition, the transcriptional organization of these genes was examined and primer extension analysis identified a single iron-regulated bidirectional promoter between the P43 gene and the fepDGC operon. (Mol Microbiol 1991 Jun;5(6):1415-28 Nucleotide sequence and genetic organization of the ferric enterobactin transport system: homology to other periplasmic binding protein-dependent systems in Escherichia coli. Shea CM, McIntosh MA PMID: 1838574, UI: 92157868). The fepD-C operon and b0591 were divergently transcribed from a 110bp region containing a binding site for the repressor protein Fur. (Mol Microbiol 1991 Jun;5(6):1405-13 Organization of genes encoding membrane proteins of the Escherichia coli ferrienterobactin permease. Chenault SS, Earhart CF PMID: 1787794, UI: 92157867). In Escherichia coli, FepB is a periplasmic protein required for uptake of iron complexed to its endogenously-synthesized siderophore enterobactin (Ent). The results demonstrated that FepB can bind ferrienterobactin (FeEnt), thereby definitely placing FeEnt transport in the periplasmic permease category of transport systems, and that the LppOmpA localization vehicle can be used with periplasmic binding proteins. (Microbiology 1995 Jul;141 ( Pt 7):1647-54 Escherichia coli periplasmic protein FepB binds ferrienterobactin. Stephens DL, Choe MD, Earhart CF. PMID: 7551033, UI: 96004464).

5.FhuA is an Escherichia coli outer membrane protein which transports the ferric siderophore ferrichrome and is the receptor for phage T5, phi 80 and T1 and for colicin M. (Biochimie 1998 May-Jun;80(5-6):363-9 FhuA, an Escherichia coli outer membrane protein with a dual function of transporter and channel which mediates the transport of phage DNA.Bonhivers M, Plancon L, Ghazi A, Boulanger P, le Maire M, Lambert O, Rigaud JL, Letellier L PMID: 9782377, UI: 98455697).The Escherichia coli fhu operon, composed of the fhuA, C, D, and B genes, is essential for the utilization of ferric siderophores of the hydroxamate type and for the uptake of the antibiotic albomycin. The experimental evidence shows that transport across the outer membrane (via FhuA, TonB, ExbB, D) rather than transport across the cytoplasmic membrane (via FhuC, D, B) was rate limiting. The stoichiometry of the components involved in the uptake of iron(III) hydroxamates seems to be important for proper functioning. (Mol Gen Genet 1998 Apr;258(1-2):156-65 Transport activity of FhuA, FhuC, FhuD, and FhuB derivatives in a system free of polar effects, and stoichiometry of components involved in ferrichrome uptake. Mademidis A, Koster W PMID: 9613584, UI: 98273640).