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Chapter 22 :

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Abstract:

The genus encompasses a diverse group of gram-positive, nonsporulating bacteria and includes both pathogenic and nonpathogenic species. is the predominant human pathogen in this group, and almost all of the research that has examined iron transport and iron-regulated systems in the corynebacteria was done with this species. The currently licensed toxoid vaccine neutralizes the severe symptoms associated with toxin activity, and its widespread use is responsible for the reduction of disease in much of the world. Since much of the research on is directed toward diphtheria toxin, it is perhaps not surprising that the first report to provide evidence of an iron transport system was made while investigating diphtheria toxin regulation. The mutations that are responsible for the defect in biosynthesis of the siderophore in HC6 and PW8 have not been mapped, and the genes required for siderophore synthesis have not been identified. Pathogenic species of and were recently shown to encode heme oxygenases involved in the removal of iron from heme. Sequence analysis of the allele from three of the HC1 heme utilization mutants revealed that they all carried point mutations in the gene. The heme activation of the promoter in requires the sensor kinase component, ChrS, which is proposed to be involved in the detection of heme or a signal generated in response to heme.

Citation: Schmitt M. 2004. , p 344-359. In Crosa J, Mey A, Payne S, Iron Transport in Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555816544.ch22

Key Concept Ranking

Two-Component Signal Transduction Systems
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Figures

Image of FIGURE 1
FIGURE 1

Genetic map of the operon and locations of mutations. The function of the predicted product encoded by each gene is indicated below the gene designation, and the size of the product is indicated in amino acids (aa). A DtxR-binding site (DBS) overlaps the promoter for the operon, and an arrow indicates the direction of transcription. The locations of the various point mutations in the HC mutants and the resulting amino acid changes caused by the mutations are indicated below the genetic map.

Citation: Schmitt M. 2004. , p 344-359. In Crosa J, Mey A, Payne S, Iron Transport in Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555816544.ch22
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Image of FIGURE 2
FIGURE 2

Genetic map of the heme transport locus. The products of the , , and genes are homologous to ABC heme transport proteins. A function for the proteins encoded by or has not been demonstrated. The size of the predicted products encoded by each gene in the operon is indicted in amino acids (aa). A DtxR-binding site (DBS) overlaps the promoter for the operon, and an arrow indicates the direction of transcription. The vector integration mutation in the gene is indicated by a triangle below the genetic map.

Citation: Schmitt M. 2004. , p 344-359. In Crosa J, Mey A, Payne S, Iron Transport in Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555816544.ch22
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Image of FIGURE 3
FIGURE 3

Comparison of heme utilization between gram-negative and gram-positive bacteria (). In gram-negative bacteria, heme is transported across the outer membrane by a TonB-dependent outer membrane heme receptor. In the periplasm, heme binds to a periplasmic binding protein, which delivers heme to a cytoplasmic membrane permease. The permease facilitates the transport of heme into the cytosol in an energy-requiring process that involves the hydrolysis of ATP by an ATPase located on the cytosolic side of the plasma membrane. The mechanism of iron extraction from heme is thought to involve a heme oxygenase in and species; however, many other gram-negative bacteria do not appear to possess heme oxygenases, and the mechanism of iron removal for these organisms is not known. In gram-positive bacteria such as , heme is proposed to bind to the HmuT lipoprotein at the cell surface. Heme is then transported into the cytosol by the HmuU permease, which utilizes energy from the hydrolysis of ATP provided by the ATPase activity of the HmuV protein. Once inside the cell, the heme oxygenase activity of HmuO degrades the heme, which results in the release of iron. OM, outer membrane; PP, periplasm; CM, cytoplasmic membrane.

Citation: Schmitt M. 2004. , p 344-359. In Crosa J, Mey A, Payne S, Iron Transport in Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555816544.ch22
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Image of FIGURE 4
FIGURE 4

Heme-dependent activation of . It is proposed that the sensor kinase protein, ChrS, detects extracellular heme through its N-terminal region, which is predicted to contain at least four transmembrane helices (indicated by diagonally striped ovals) and two extracellular loop regions. The detection of heme or hemoglobin is thought to result in autophosphorylation of ChrS at a conserved His residue (H) located within the cytosolic histidine kinase domain (boxed region). The phosphoryl group (P) is then transferred to a conserved Asp residue (D) on ChrA. Phosphorylation activates the DNA-binding function of ChrA and allows ChrA to bind upstream of the promoter and activate transcription. Under high-iron conditions, DtxR represses the transcription of the promoter. The gene, therefore, is optimally expressed in low-iron environments in the presence of heme. The HmuO protein is proposed to degrade the cytosolic heme and liberate the heme-bound iron.

Citation: Schmitt M. 2004. , p 344-359. In Crosa J, Mey A, Payne S, Iron Transport in Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555816544.ch22
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Image of FIGURE 5
FIGURE 5

Functional domains of DtxR and amino acid residues associated with metal-binding sites 1 and 2 (mbs 1 and mbs 2). H-T-H, helix-turn-helix.

Citation: Schmitt M. 2004. , p 344-359. In Crosa J, Mey A, Payne S, Iron Transport in Bacteria. ASM Press, Washington, DC. doi: 10.1128/9781555816544.ch22
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References

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