Phosphohistidines in bacterial signaling.

The movement of Gram-negative bacteria in response to nutrients in the environment is driven by two interlinked chemotaxis systems, the methyl-accepting chemotaxis protein (MCP)-mediated pathway, and the phosphoenolpyruvate: sugar phosphotransferase (PTS)-mediated pathway. The physical link connecting the two systems is unclear, but the common utilization of histidine-containing phosphocarrier proteins is an intriguing similarity. The recent structure determinations of several proteins from the PTS-mediated pathway, the phosphotransfer domain from the kinase CheA of the MCP-mediated chemotaxis pathway, and a homologous kinase, ArcB, enable the comparison of the histidine active sites of these systems. Overall, the tertiary folds of the proteins are quite different, as are the structural details of the histidine active sites within the proteins, and therefore there is not an obvious structural homolog via which the two pathways communicate, despite their similar chemical mechanisms.