The Fur repressor controls transcription of iron-activated and -repressed genes in Helicobacter pylori

The ferric uptake regulator (Fur) protein is known to act as a Fe2+-dependent transcriptional repressor of bacterial promoters. Here, we show that, in Helicobacter pylori, Fur can mediate the regulation of iron-activated genes in contrast to classical Fur regulation, in which iron acts as a co-repressor. Inactivation of the fur gene in the chromosome of H. pylori resulted in the derepression of a 19 kDa protein that was identified by N-terminal sequencing as the non-haem-containing ferritin (Pfr). Growth of the wild-type H. pylori strain on media treated with increasing concentrations of FeSO4 resulted in induction of transcription from the Ppfr promoter and, conversely, depletion of iron resulted in repression of Ppfr, indicating that this promoter is iron activated. In the fur mutant, the Ppfr promoter is constitutively highly expressed and no longer responds to iron, indicating that the Fur protein mediates this type of iron regulation. Footprinting analysis revealed that Fur binds to the Ppfr promoter region and that Fe2+ decreases the efficiency of binding. In contrast, Fe2+ increased the affinity of Fur for a classical Fur-regulated promoter, the iron-repressed frpB gene promoter. To our knowledge, this is the first evidence of direct interaction between the Fur protein and the promoter of an iron-activated (-derepressed) gene. Our results support a model in which the iron status of the Fur protein differentially alters its affinity for operators in either iron-repressed or iron-activated genes.

Iron-dependent transcription of the frpB gene of Helicobacter pylori is controlled by the Fur repressor protein

We have overexpressed and purified the Helicobacter pylori Fur protein and analyzed its interaction with the intergenic regions of divergent genes involved in iron uptake (frpB and ceuE) and oxygen radical detoxification (katA and tsaA). DNase I footprint analysis showed that Fur binds specifically to a high-affinity site overlapping the P(frpB) promoter and to low-affinity sites located upstream from promoters within both the frpB-katA and ceuE-tsaA intergenic regions. Construction of an isogenic fur mutant indicated that Fur regulates transcription from the P(frpB) promoter in response to iron. In contrast, no effect by either Fur or iron was observed for the other promoters.

Regulation of transcription in Helicobacter pylori: simple systems or complex circuits?

A common strategy used by both Gram-negative and Gram-positive bacterial pathogens is based on the synchronisation of virulence gene expression using a variety of regulatory systems and networks to overcome host defence. During the last decade an exponentially growing number of studies on Helicobacter pylori, a human pathogen associated with diverse stomach diseases, have mainly focussed on the elucidation of mechanisms and functions of virulence factors. A subset of these studies were focussed on the molecular mechanisms regulating gene transcription in H. pylori with the aim of understanding the profound physiological changes that this pathogen, as well as other bacteria, undergoes during infection. Despite the limited number of putative regulatory proteins, as deduced from genome sequence analyses, evidence is accumulating for the existence of new and complex circuits regulating gene transcription and virulence of this bacterium. Here we will focus on the molecular mechanisms used by H. pylori to control gene transcription.