B-255. A Hierarchy of Nickel Trafficking in Helicobacter pylori

E. L. Benanti, P. T. Chivers;
Washington Univ. Sch. of Med., St. Louis, MO.

Nickel is essential for the gastric pathogen Helicobacter pylori to successfully colonize the stomach. The two Ni2+-dependent enzymes produced by H. pylori, urease and hydrogenase, require the chaperones UreE and HypA-HypB-SlyD for the transport and insertion of Ni2+ into the enzyme active sites. H. pylori express the Ni2+-dependent transcription factor NikR to regulate many genes in response to increased intracellular Ni2+, including the repression of nixA and frpB4, which encode Ni2+-importers. NikR displays extremely high affinity (~2 pM) for Ni2+ in vitro, however NikR activation in cells under low extracellular Ni2+ conditions would repress Ni2+ import and block further enzyme assembly. To determine if Ni2+-binding chaperones or enzymes affect NikR activation in H. pylori cells, we compared nixA and frpB4 transcript levels, 63Ni-accumulation and urease activity in wild-type or mutant strains lacking Ni2+-binding chaperones or enzymes. A subset of the mutant strains displayed an increase in NikR activity relative to wild-type cells under Ni2+-limiting conditions. The same subset of mutant strains displayed a decrease in 63Ni-accumulation and low levels of urease activity under excess Ni2+ conditions. These results demonstrate that the chaperones UreE, HypB and HypA-SlyD compete with NikR for intracellular Ni2+. The data also establish that the Ni2+-enzyme pathways inhibit NikR activity under Ni2+-limiting conditions, thereby creating a hierarchy of Ni2+-trafficking in H. pylori. This study provides new insight into H. pylori Ni2+ physiology and also describes a useful model system for understanding transition metal homeostasis in all cells.