Phosphorylation-independent activity of atypical response regulators of Helicobacter pylori

The genome of the gastric pathogen Helicobacter pylori harbors a remarkably low number of regulatory genes, including three and five open reading frames encoding two-component histidine kinases and response regulators, respectively, which are putatively involved in transcriptional regulation. Two of the response regulator genes, hp1043 and hp166, proved to be essential for cell growth, and inactivation of the response regulator gene hp1021 resulted in a severe growth defect, as indicated by a small-colony phenotype. The sequences of the receiver domains of response regulators HP1043 and HP1021 differ from the consensus sequence of the acidic pocket of the receiver domain which is involved in the phosphotransfer reaction from the histidine kinase to the response regulator. Using a genetic complementation system, we demonstrated that the function of response regulator HP166, which is essential for cell growth, can be provided by a mutated derivative carrying a D52N substitution at the site of phosphorylation. We found that the atypical receiver sequences of HP1043 and HP1021 are not crucial for the function of these response regulators. Phosphorylation of the receiver domains of HP1043 and HP1021 is not needed for response regulator function and may not occur at all. Thus, the phosphorylation-independent action of these regulators differs from the well-established two-component paradigm.