Transcriptional Regulation of the Nickel and Iron Metabolism in Helicobacter pylori

Up to 50 % of the world's population is infected with Helicobacter pylori. Colonization of the mucus layer of the human stomach by H. pylori, is lifelong unless treated with antibiotics (26). H. pylori, which is a neutralophilic bacterium, survives in the mucus layer of the human stomach with the help of the enzyme urease. Urease is an enzyme that converts urea into ammonium and carbon dioxide, thereby keeping the intracellular and periplasmic pH at neutral. It is estimated that up to 10% of the whole cell protein consists of this nickelcofactored enzyme (19). The nickel necessary to activate the urease is thought to come from foodsources, such as nuts, tea and cereals, which are rich in nickel (1). Metal ions like nickel or iron can be dangerous for bacteria, as they can react with oxygen in order to create reactive oxygen species that in turn can destroy macromolecules like nucleic acids, proteins and cell wall components (27). Therefore, the bacterial metal metabolism has to be tightly regulated. In H. pylori, regulatory proteins are scarce. Only two metal-regulatory proteins are known, the ferric uptake regulator Fur (4), and the nickel responsive regulator NikR (31). Fur is a regulatory protein that can sense and bind intracellular ferrous ions, and subsequently displays iron-dependent binding to conserved promoter sequences (Fur boxes) of its target genes (17). The classical regulation is repression of iron uptake genes in iron-replete conditions (15, 17, 33). Unlike all other Fur homologs known so far, H. pylori Fur can also bind to Fur-boxes in an iron free form (apo-Fur), as was shown for pfr (16) and sodB (Chapter 3). The second metal-dependent regulatory protein is NikR, the nickel responsive regulator, which belongs to the family of Ribbon-Helix-Helix regulatory proteins (9). NikR is directly involved in the regulation of acid resistance via urease and nickel-uptake (Chapter 4), and was previously demonstrated to mediate regulation of the ferric uptake regulator Fur (7, 12, 29). The aim of this thesis was to gain further insight into the transcriptional regulation the ferric uptake regulator Fur and the nickel responsive regulator NikR.

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