Differential regulation of the three fecA metal transporter genes in Helicobacter pylori

Balancing metals uptake is essential for maintaining a proper intracellular metal concentration. Here we report about the transcriptional control operated by the two metal-responsive and main global regulators of Helicobacter pylori G27, Fur (iron-dependent ferric uptake regulator) and NikR (nickel-responsive regulator), on the three copies of the fecA genes present in this species. The fecA genes share approximately 50% overall sequence similarity to the E. coli outer-membrane ferric citrate transporter homologue, however their expression differs significantly from the fecA of E. coli, with the vegetative sigma-80 RNA polymerase being required for transcription, instead of an alternative sigma factor. By following the patterns of expression throughout growth and in response to nickel, iron and a metal chelator, we found that the expression of the three fecA genes is temporally regulated, responds differently to metals and is selectively controlled by either one of the two regulators. fecA1 is expressed at constant level throughout growth and its expression is iron-sensitive; the expression of fecA2 is mainly off, with minor expression coming up in late exponential phase. On the other hand, the expression of fecA3 is maximal in early exponential phase, but gradually decreases with time and is selectively repressed by nickel. The direct role of Fur and NikR was studied both in vitro, by mapping the binding sites of each regulator on the promoter regions via DNaseI footprinting analysis, and in vivo by primer extension analyses of the fecA transcripts in fur and nikR deletion strains. The resulting picture suggests dedicated feedback regulatory circuits, where each fecA homologue is likely to be required for the selective import of metals and its expression is specifically controlled at transcriptional level by the metal responsive regulator.