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.