Helicobacter hepaticus NikR controls urease and hydrogenase activities via the NikABDE and HH0418 putative nickel import proteins.

Helicobacter hepaticus open reading frame HH0352 was identified as a nickel-responsive regulator NikR. The gene was disrupted by insertion of an erythromycin resistance cassette. The H. hepaticus nikR mutant had five- to sixfold higher urease activity and at least twofold greater hydrogenase activity than the wild-type strain. However, the urease apo-protein levels were similar in both the wild-type and the mutant, suggesting the increase in urease activity in the mutant was due to enhanced Ni-maturation of the urease. Compared with the wild-type strain, the nikR strain had increased cytoplasmic nickel levels. Transcription of nikABDE (putative inner membrane Ni transport system) and hh0418 (putative outer membrane Ni transporter) was nickel- and NikR-repressed. Electrophoretic mobility shift assays (EMSAs) revealed that purified HhNikR could bind to the nikABDE promoter (P(nikA)), but not to the urease or the hydrogenase promoter; NikR-P(nikA) binding was enhanced in the presence of nickel. Also, qRT-PCR and EMSAs indicated that neither nikR nor the exbB-exbD-tonB were under the control of the NikR regulator, in contrast with their Helicobacter pylori homologues. Taken together, our results suggest that HhNikR modulates urease and hydrogenase activities by repressing the nickel transport/nickel internalization systems in H. hepaticus, without direct regulation of the Ni-enzyme genes (the latter is the case for H. pylori). Finally, the nikR strain had a two- to threefold lower growth yield than the parent, suggesting that the regulatory protein might play additional roles in the mouse liver pathogen.

Roles of His-rich hpn and hpn-like proteins in Helicobacter pylori nickel physiology.

Individual gene-targeted hpn and hpn-like mutants and a mutant with mutations in both hpn genes were more sensitive to nickel, cobalt, and cadmium toxicity than was the parent strain, with the hpn-like strain showing the most metal sensitivity of the two individual His-rich protein mutants. The mutant strains contained up to eightfold more urease activity than the parent under nickel-deficient conditions, and the parent strain was able to achieve mutant strain activity levels by nickel supplementation. The mutants contained 3- to 4-fold more and the double mutant about 10-fold more Ni associated with their total urease pools, even though all of the strains expressed similar levels of total urease protein. Hydrogenase activities in the mutants were like those in the parent strain; thus, hydrogenase is fully activated under nickel-deficient conditions. The histidine-rich proteins appear to compete with the Ni-dependent urease maturation machinery under low-nickel conditions. Upon lowering the pH of the growth medium from 7.3 to 5, the wild-type urease activity increased threefold, but the activity in the three mutant strains was relatively unaffected. This pH effect was attributed to a nickel storage role for the His-rich proteins. Under low-nickel conditions, the addition of a nickel chelator did not significantly affect the urease activity of the wild type but decreased the activity of all of the mutants, supporting a role for the His-rich proteins as Ni reservoirs. These nickel reservoirs significantly impact the active urease activities achieved. The His-rich proteins play dual roles, as Ni storage and as metal detoxification proteins, depending on the exogenous nickel levels.

Nickel-binding and accessory proteins facilitating Ni-enzyme maturation in Helicobacter pylori.

Helicobacter pylori colonizes the human gastric mucosa and this can lead to chronic gastritis, peptic and duodenal ulcers, and even gastric cancers. The bacterium colonizes over one-half of the worlds population. Nickel plays a major role in the bacteriums colonization and persistence attributes as two nickel enzyme sinks obligately contain the metal. Urease accounts for up to 10% of the total cellular protein made and is required for initial colonization processes, and the hydrogen oxidizing hydrogenase provides the bacterium a high-energy substrate yielding low potential electrons for energy generation. A battery of accessory proteins are needed for maturation or activation of each of the apoenzymes. These include Ni-chaperones and GTPases, some of which are unique to each Ni-enzyme and others that are individually required for maturation of both the Ni-enzymes. H. pylori's need for some conventional hydrogenase maturation proteins playing roles in urease maturation may have to do with the poor nickel-sequestering ability of the UreE urease maturation protein compared to other systems. H. pylori also possesses a NixA nickel specific permease, a nickel dependent regulator (NikR), a recently identified nickel efflux system (CznABC), and a histidine-rich heat shock protein, HspA. Based on mutant analysis approaches all these proteins have roles in nickel homeostasis, in urease expression, and in host colonization. The His-rich putative nickel storage proteins Hpn and Hpn-like play roles in nickel detoxification and may influence the levels of Ni-activated urease that can be achieved.