Contribution of dppA to Urease Activity in Helicobacter pylori 26695

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Abstract
The gastric pathogen Helicobacter pylori produces urease in amounts up to 10% of its cell protein. This enzyme, which catalyzes the hydrolysis of urea to ammonia and carbon dioxide, protects the bacterium from gastric acid. Urease, a nickel metalloenzyme, requires active uptake of nickel ions from the environment to maintain its activity. NixA is a nickel transport protein that resides in the cytoplasmic membrane. Mutation of nixA significantly reduces but does not abolish urease activity, strongly suggesting the presence of a second transporter. We postulated that the dipeptide permease (dpp) genes that are homologous to the nik operon of Escherichia coli could be a second nickel transporter. The predicted Dpp polypeptides DppA, DppC, and DppD of H. pylori share approximately 40%, 53%, and 56% amino acid sequence identity with their respective E. coli homologs. A mutation in dppA, constructed by insertional inactivation with a chloramphenicol resistance cassette, was introduced by allelic exchange into H. pylori strain 26695. When compared to the parental strain, urease activity was not decreased in a dppA mutant. DppA does not contribute to the synthesis of catalytically active urease in H. pylori 26695 and is likely not a nickel importer in H. pylori.