Abstract
The NikR protein is a nickel-dependent regulatory protein which is a member of the ribbon-helix-helix family of transcriptional regulators. The gastric pathogen
Helicobacter pylori
expresses a NikR ortholog, which was previously shown to mediate regulation of metal metabolism and urease expression, but the mechanism governing the diverse regulatory effects had not been described until now. In this study it is demonstrated that NikR can regulate
H. pylori
nickel metabolism by directly controlling transcriptional repression of NixA-mediated nickel uptake and transcriptional induction of urease expression. Mutation of the nickel uptake gene
nixA
in an
H. pylori
26695
nikR
mutant restored the ability to grow in Brucella media supplemented with 200 μM NiCl
2
but did not restore nickel-dependent induction of urease expression. Nickel-dependent binding of NikR to the promoter of the
nixA
gene resulted in nickel-repressed transcription, whereas nickel-dependent binding of NikR to the promoter of the
ureA
gene resulted in nickel-induced transcription. Subsequent analysis of NikR binding to the
nixA
and
ureA
promoters showed that the regulatory effect was dependent on the location of the NikR-recognized binding sequence. NikR recognized the region from −13 to +21 of the
nixA
promoter, encompassing the +1 and −10 region, and this binding resulted in repression of
nixA
transcription. In contrast, NikR bound to the region from −56 to −91 upstream of the
ureA
promoter, resulting in induction of urease transcription. In conclusion, the NikR protein is able to function both as a repressor and as an activator of gene transcription, depending on the position of the binding site.