Recognition of ferric catecholates by FepA

Iron acquisition systems for ferric hydroxamates, haemin and haemoglobin in Listeria monocytogenes

Listeria monocytogenes is a Gram-positive bacterium that causes severe opportunistic infections in humans and animals. We biochemically characterized, for the first time, the iron uptake processes of this facultative intracellular pathogen, and identified the genetic loci encoding two of its membrane iron transporters. Strain EGD-e used iron complexes of hydroxamates (ferrichrome and ferrichrome A, ferrioxamine B), catecholates (ferric enterobactin, ferric corynebactin) and eukaryotic binding proteins (transferrin, lactoferrin, ferritin, haemoglobin). Quantitative determinations showed 10-100-fold lower affinity for ferric siderophores (Km approximately 1-10 nM) than Gram-negative bacteria, and generally lower uptake rates. Vmax for [59Fe]-enterobactin (0.15 pMol per 10(9) cells per minute) was 400-fold lower than that of Escherichia coli. For [59Fe]-corynebactin, Vmax was also low (1.2 pMol per 10(9) cells per minute), but EGD-e transported [59Fe]-apoferrichrome similarly to E. coli (Vmax=24 pMol per 10(9) cells per minute). L. monocytogenes encodes potential Fur-regulated iron transporters at 2.031 Mb (the fur-fhu region), 2.184 Mb (the feo region), 2.27 Mb (the srtB region) and 2.499 Mb (designated hupDGC region). Chromosomal deletions in the fur-fhu and hupDGC regions diminished iron uptake from ferric hydroxamates and haemin/haemoglobin respectively. In the former locus, deletion of fhuD (lmo1959) or fhuC (lmo1960) strongly reduced [59Fe]-apoferrichrome uptake. Deletion of hupC (lmo2429) eliminated the uptake of haemin and haemoglobin, and decreased the virulence of L. monocytogenes 50-fold in mice. Elimination of srtB region genes (Deltalmo2185, Deltalmo2186, Deltalmo2183), both sortase structural genes (DeltasrtB, DeltasrtA, DeltasrtAB), fur and feoB did not impair iron transport. However, deletion of bacterioferritin (Deltafri, lmo943; 0.97 Mb) decreased growth and altered iron uptake: Vmax of [59Fe]-corynebactin transport tripled in this strain, whereas that of [59Fe]-apoferrichrome decreased 20-fold.

The svpA-srtB locus of Listeria monocytogenes: fur-mediated iron regulation and effect on virulence

In Listeria monocytogenes the promoter region of the svpA-srtB locus contains a well-conserved Fur box. We characterized the iron-regulation of this locus: real-time polymerase chain reaction analyses and anti-SvpA immunoblots showed that, in response to iron deprivation svpA transcription and SvpA production markedly increased (80-fold and 10-fold respectively), when initiated by either the addition of the iron chelator 2,2'-bipyridyl to BHI media, or by growth in iron-restricted minimal media. Green fluorescent protein (GFP) reporter constructs also showed increased activity of the svpA-srtB promoter in Escherichia coli (37-fold) and in L. monocytogenes (two- to threefold) when the bacteria were grown in iron-deficient conditions. A Deltafur mutant of L. monocytogenes constitutively synthesized SvpA, as well as GFP fused to the svpA-srtB promoter. Cellular fractionation data revealed that in iron-rich media wild-type SvpA was exclusively secreted to the culture supernatant. However, both the Deltafur derivative and wild-type L. monocytogenes grown in iron-deficient media anchored a fraction of the SvpA proteins (approximately 5%) to peptidoglycan, and produced a lower-molecular weight, wholly secreted form of SvpA. Together these data establish that iron availability controls transcription of the svpA-srtB locus (through Fur-mediated regulation), and attachment of SvpA to the cell wall (through SrtB-mediated covalent linkage). SvpA bears homology to IsdC, a haemin-binding protein of Staphylococcus aureus, and haemin bound to SvpA in solution. However, site-directed deletions of four structural genes and the promoter of the svpA-srtB locus did not impair haemin, haemoglobin or ferrichrome utilization in nutrition tests. We did not find strong evidence to support the notion that the svpA-srtB locus participates in haemin acquisition, as was reported for the homologous isd operon of S. aureus. Furthermore, the svpA-srtB mutant strains showed no significant attenuation of virulence in an intravenous mouse model system, but we found that the mutations reduced the persistence of L. monocytogenes in murine liver, spleen and intestines after oral administration.

In vitro characterization of IroB, a pathogen-associated C-glycosyltransferase

Pathogenic strains of Escherichia coli and Salmonella enterica modify the tricatecholic siderophore enterobactin (Ent) by glucosylation of three aryl carbon atoms, a process controlled by the iroA locus [Hantke, K., Nicholson, G., Rabsch, W. & Winkelmann, G. (2003) Proc. Natl. Acad. Sci. USA 100, 3677-3682]. Here, we report the purification of the IroB protein and its characterization as the Ent C-glucosyltransferase. IroB transfers glucosyl groups from uridine-5'-diphosphoglucose to C5 of one, two, or three of the 2,3-dihydroxybenzoyl units of Ent to yield monoglucosyl-C-Ent (MGE), diglucosyl-C-Ent (DGE), and triglucosyl-C-Ent (TGE). DGE, also known as salmochelin S4, and macrolactone-opened derivatives have been isolated from the culture broths of S. enterica and uropathogenic E. coli [Bister, B., Bischoff, D., Nicholson, G. J., Valdebenito, M., Schneider, K., Winkelmann, G., Hantke, K. & Sussmuth, R. D. (2004) Biometals 17, 471-481], but MGE and TGE have not been reported previously. IroB has a k(cat) of approximately 10 min(-1) for the first C-glucosylation and is distributive, with sequential conversion and buildup of MGE and then DGE. The C5 to C1' regio-selectivity of the 2,3-dihydroxybenzoyl-glucose linkage at all three rings of TGE suggests a C5 carbanion, para to the C2 phenolate oxygen, as the carbon nucleophile in this novel enzymatic C-glucosylation.