Subsystem: Ferrous iron transporter EfeUOB, low-pH-induced

This subsystem's description is:

A novel transporter responsible for uptake of ferrous iron under conditions of iron limitation and low pH (cryptic in E. coli lab strains, but active in E.coli O157:H7).

The predominant environmental form of iron is the poorly soluble ferric form (10-18 M at pH 7.0). Relatively soluble (0.1 M at pH 7.0) ferrous iron is mainly restricted to anaerobic/microaerobic niches or low pH environments. Low solubility of ferric iron leads to problems of poor bioavailability. In addition, iron can cause toxicity in the presence of oxygen due to its tendency to engage in Fenton chemistry generating highly-reactive free-radical species. Consequently, organisms have developed various strategies that allow them to acquire sufficient iron for growth whilst avoiding any deleterious effects caused by iron toxicity (e.g. siderophore-dependent transporters)

In contrast to ferric iron (Fe3+), ferrous iron (Fe2+) can be directly imported using FeoAB transporters, which are thought to function under anaerobic-microaerophilic conditions. In addition, metal-type ABC transporters [e.g. SitABCD of Salmonella typhimurium] and NRAMP-like transporters [e.g. MntH of Escherichia coli] also can translocate Fe2+.

Recently, a new type of ferrous-iron transporter, designated EfeUOB, has been identified in E. coli. It consists of three protein components (EfeUOB) and acts mainly at low pH (Cao et al. 2007). Homologous systems are present in many other bacteria. The efeUOB operon is induced at low pH upon phosphorylation of the CpxR component of the CpxAR two-component response regulator (for copper). Expression increases in response to iron-depleted conditions in a Fe2+-Fur (Iron Uptake Regulator)-dependent manner. Also repressed by luxS. Since these regulators play roles elsewhere in the cell, and are part of other subsystems, I will not include them in this one.

In some Streptococci, the efe operon is interrupted by remnants of a transposase, indicating an insertion event.

For more information, please check out the description and the additional notes tabs, below

DiagramFunctional RolesSubsystem SpreadsheetDescriptionAdditional Notes 

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*EfeUEfeOEfeB
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A novel transporter responsible for uptake of ferrous iron under conditions of iron limitation and low pH (cryptic in E. coli lab strains, but active in E.coli O157:H7).

The predominant environmental form of iron is the poorly soluble ferric form (10-18 M at pH 7.0). Relatively soluble (0.1 M at pH 7.0) ferrous iron is mainly restricted to anaerobic/microaerobic niches or low pH environments. Low solubility of ferric iron leads to problems of poor bioavailability. In addition, iron can cause toxicity in the presence of oxygen due to its tendency to engage in Fenton chemistry generating highly-reactive free-radical species. Consequently, organisms have developed various strategies that allow them to acquire sufficient iron for growth whilst avoiding any deleterious effects caused by iron toxicity (e.g. siderophore-dependent transporters)

In contrast to ferric iron (Fe3+), ferrous iron (Fe2+) can be directly imported using FeoAB transporters, which are thought to function under anaerobic-microaerophilic conditions. In addition, metal-type ABC transporters [e.g. SitABCD of Salmonella typhimurium] and NRAMP-like transporters [e.g. MntH of Escherichia coli] also can translocate Fe2+.

Recently, a new type of ferrous-iron transporter, designated EfeUOB, has been identified in E. coli. It consists of three protein components (EfeUOB) and acts mainly at low pH (Cao et al. 2007). Homologous systems are present in many other bacteria. The efeUOB operon is induced at low pH upon phosphorylation of the CpxR component of the CpxAR two-component response regulator (for copper). Expression increases in response to iron-depleted conditions in a Fe2+-Fur (Iron Uptake Regulator)-dependent manner. Also repressed by luxS. Since these regulators play roles elsewhere in the cell, and are part of other subsystems, I will not include them in this one.

In some Streptococci, the efe operon is interrupted by remnants of a transposase, indicating an insertion event.
Protein components:

EfeU, an integral inner membrane protein with seven transmembrane segments, is an oxidase-dependent ferrous iron permease. In E. coli K-12 the gene is disrupted by a frameshift.

EfeO is a periplasmic and contains a predicted N-terminal signal sequence suggesting export to the periplasm in a Sec-dependent manner. EfeO consists of a C-terminal peptidase-M75 (M75) domain (~225 residues) and an N-terminal domain of ~100 residues that appears to resemble the copper-containing cupredoxins (but not in all EfeO homologs). Function is unknown

EfeB is a dimeric, non-covalently bound heme-containing peroxidase enzyme of the DyP-type peroxidase family, and is a substrate of the twin arginine translocation (Tat) system. The heme cofactor is FeIII-protoporphyrin IX. EfeB may act to reduce Fe3+ to Fe2+ for transport or to oxidize Fe2+ during transport; however its natural substrate is unknown.

Recently an unusual function has been proposed for EfeB: deferrochelatase of heme (Letoffe et al., 2009). EfeB (and YfeX) were shown in this work to promote iron extraction from heme preserving the tetrapyrrole ring intact. The authors propose that the physiological function of EfeB and YfeX is to retrieve iron from exogenous heme


References:

Cao J, Woodhall MR, Alvarez J, Cartron ML, Andrews SC. 2007. EfeUOB (YcdNOB) is a tripartite, acid-induced and CpxAR-regulated, low-pH Fe2+ transporter that is cryptic in Escherichia coli K-12 but functional in E. coli O157:H7. Mol Microbiol, 65(4):857-75.

Grosse C, Scherer J, Koch D, Otto M, Taudte N, Grass G. 2006. A new ferrous iron-uptake transporter, EfeU (YcdN), from Escherichia coli. Mol Microbiol, 62(1):120-31

Létoffé S, Heuck G, Delepelaire P, Lange N, Wandersman C. 2099. Bacteria capture iron from heme by keeping tetrapyrrol skeleton intact. Proc Natl Acad Sci U S A, 106(28):11719-24.

Rajasekaran MB, Nilapwar S, Andrews SC, Watson KA. 2009. EfeO-cupredoxins: major new members of the cupredoxin superfamily with roles in bacterial iron transport. Biometals. 2009 Aug 23. [Epub ahead of print]

Sturm A, Schierhorn A, Lindenstrauss U, Lilie H, Brüser T. 2006. YcdB from Escherichia coli reveals a novel class of Tat-dependently translocated hemoproteins. J Biol Chem, 281(20):13972-8

Wang L, Li J, March JC, Valdes JJ, Bentley WE. 2005. luxS-dependent gene regulation in Escherichia coli K-12 revealed by genomic expression profiling. J Bacteriol., 187(24):8350-60