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We show here that Mg 2+ acquisition by CorA is essential for Helicobacter pylori in vitro, as corA mutants did not grow in media without Mg 2+ supplementation. Complementation analysis performed with an Escherichia coli corA mutant revealed that H. pylori CorA transports nickel and cobalt in addition to Mg 2+ . However, Mg 2+ is the dominant CorA substrate, as the corA mutation affected neither cobalt and nickel resistance nor nickel induction of urease in H. pylori . The drastic Mg 2+ requirement (20 mM) of H. pylori corA mutants indicates that CorA plays a key role in the adaptation to the low-Mg 2+ conditions predominant in the gastric environment.

The reactivity of the essential element iron necessitates a concerted expression of ferritins, which mediate iron storage in a nonreactive state. Here we have further established the role of the Helicobacter pylori ferritin Pfr in iron metabolism and gastric colonization. Iron stored in Pfr enabled H. pylori to multiply under severe iron starvation and protected the bacteria from acid-amplified iron toxicity, as inactivation of the pfr gene restricted growth of H. pylori under these conditions. The lowered total iron content in the pfr mutant, which is probably caused by decreased iron uptake rates, was also reflected by an increased resistance to superoxide stress. Iron induction of Pfr synthesis was clearly diminished in an H. pylori feoB mutant, which lacked high-affinity ferrous iron transport, confirming that Pfr expression is mediated by changes in the cytoplasmic iron pool and not by extracellular iron. This is well in agreement with the recent discovery that iron induces Pfr synthesis by abolishing Fur-mediated repression of pfr transcription, which was further confirmed here by the observation that iron inhibited the in vitro binding of recombinant H. pylori Fur to the pfr promoter region. The functions of H. pylori Pfr in iron metabolism are essential for survival in the gastric mucosa, as the pfr mutant was unable to colonize in a Mongolian gerbil-based animal model. In summary, the pfr phenotypes observed give new insights into prokaryotic ferritin functions and indicate that iron storage and homeostasis are of extraordinary importance for H. pylori to survive in its hostile natural environment.