The phosphoryl transfer domain of UhpB interacts with the response regulator UhpA.

Bacterial two-component regulatory systems control the expression of target genes through regulated changes in protein phosphorylation. Signal reception alters the ability of a membrane-bound histidine kinase (HK) protein to transfer phosphate from ATP to a highly conserved histidine residue. The transfer of phosphate from the histidine to an aspartate residue on the cognate response regulator (RR) changes the ability of the latter protein to bind to target DNA sequences and to alter gene transcription. UhpB is the HK protein which controls production of the sugar phosphate transporter UhpT. Elevated expression of full-length UhpB or of a soluble hybrid protein, GST-Bc, which is glutathione S-transferase (GST) fused to the cytoplasmic C-terminal portion of UhpB, results in complete blockage of uhpT expression in a uhp(+) strain. This dominant-negative interference could result from the ability of GST-Bc to bind and sequester the RR UhpA and to accelerate its dephosphorylation. The portion of GST-Bc responsible for the interference phenotype was localized using truncation, linker insertion, and point mutations to the region between residues 293 and 366 flanking His-313, the putative site of autophosphorylation. Point mutations which allow GST-Bc to activate uhpT expression or which relieve the interference phenotype were obtained at numerous sites throughout this region. This region of UhpB is related to the phosphoryl transfer domain of EnvZ, which forms half of an interdimer four-helix bundle and is responsible for dimerization of its cytoplasmic domain. The expression of GST fusion proteins carrying the corresponding portions of EnvZ strongly interfered with the activation of porin gene expression by OmpR. The GST-Bc protein accelerated dephosphorylation of P-UhpA. Reverse transfer of phosphate from P-UhpA to GST-Bc was observed in the presence of the metal chelator EDTA and depended on the presence of His-313. Phosphate transfer from P-UhpA to the liberated phosphoryl transfer domain also occurred. Taken together, these results indicate that the phosphoryl transfer-dimerization domain of UhpB participates in the specific binding of UhpA, in the control of autokinase activity, and in the dephosphorylation of P-UhpA.