COUPLING THE PHOSPHOTRANSFERASE SYSTEM AND THE METHYL-ACCEPTING CHEMOTAXIS PROTEIN-DEPENDENT CHEMOTAXIS SIGNALING PATHWAYS OF ESCHERICHIA-COLI

Chemotactic responses in Escherichia coli are typically mediated by tr ansmembrane receptors that monitor chemoeffector levels with periplasm ic binding domains and communicate with the flagellar motors through t wo cytoplasmic proteins, CheA and CheY. CheA autophosphorylates and th en donates its phosphate to CheY, which in turn controls flagellar rot ation. E. coli also exhibits chemotactic responses to substrates that are transported by the phosphoenolpyruvate (PEP)-dependent carbohydrat e phosphotransferase system (PTS). Unlike conventional chemoreception, PTS substrates are sensed during their uptake and concomitant phospho rylation by the cell. The phosphoryl groups are transferred from PEP t o the carbohydrates through two common intermediates, enzyme I (EI) an d phosphohistidine carrier protein (HPr), and then to sugar-specific e nzymes II. We found that in mutant strains HPr-like proteins could sub stitute for HPr in transport but did not mediate chemotactic signaling . In in vitro assays, these proteins exhibited reduced phosphotransfer rates from EI, indicating that the phosphorylation state of EI might link the PTS phospho-relay to the flagellar signaling pathway. Tests w ith purified proteins revealed that unphosphorylated EI inhibited CheA autophosphorylation, whereas phosphorylated EI did not. These finding s suggest the following model for signal transduction in PTS-dependent chemotaxis. During uptake of a PTS carbohydrate, EI is dephosphorylat ed more rapidly by HPr than it is phosphorylated at the expense of PEP . Consequently, unphosphorylated EI builds up and inhibits CheA autoph osphorylation. This slows the flow of phosphates to CheY, eliciting an up-gradient swimming response by the cell.