The smaller of two overlapping cheA gene products is not essential for chemotaxis in Escherichia coli

The cheA locus ofEscherichia coliencodes two similar proteins, CheAL (654 amino acids) and CheAS (557 amino acids), which are made by initiating translation from different in-frame start sites [start(L) and start(S)]. CheALplays an essential role in chemotactic signaling. It autophosphorylates at a histidine residue (His-48) and then donates this phosphate to response regulator proteins that modulateflagellar rotation and sensory adaptation. CheAS lacks the first 97 amino acids of CheAL, including the phosphorylation site at His-48.Althoughitisunabletoautophosphorylate,CheAScanformheterodimerswithmutantCheALsubunits to restore kinase function and chemoreceptor control of autophosphorylation activity. To determine whether these or other activities of CheAS are important for chemotaxis, we constructed cheA lesions that abrogated CheASexpression.MutantsinwhichtheCheASstartcodonwaschangedfrommethioninetoisoleucine(M98I) or glutamine (M98Q) retained chemotactic ability, ranging from 50% (M98Q) to 80% (M98I) of wild-type function. These partial defects could not be alleviated by supplying CheAS from a specialized transducing phage, indicating that the lesions in CheAL—not the lack of CheAS—were responsible for the reduced chemotactic ability. In other respects, the behavior of the M98I mutant was essentially normal. Its flagellar rotationpatternwasindistinguishablefromwildtype,anditexhibitedwild-typedetectionthresholdsandpeak positions in capillary chemotaxis assays. The lack of any substantive defect in this start(S) mutant argues that CheAS makes a negligible contribution to chemotactic ability in the laboratory. Whether it has functional significance in other settings remains to be seen. The chemotactic behavior of Escherichia coli provides a model system for investigating signal transduction mechanisms atthemolecularlevel(seereference23forarecentreview).As they swim about, these cells use transmembrane chemoreceptors to monitor concentrations of beneficial or harmful chemicals in their environment. Changes in attractant or repellent levels trigger changes inflagellar rotation that promote migration in favorable directions: counterclockwise (CCW) rotation produces forward movement; clockwise (CW) rotation causes abrupt turns or tumbling episodes. The chemoreceptors communicate with the flagellar motors through a series of intracellularproteinphosphorylationreactions,muchlikethesignal transduction pathways of eukaryotes. ThecheAlocus plays a central role in this signaling cascade. It encodes a histidine kinase that autophosphorylates, using ATP as the phosphate donor (8). CheA donates its phosphate groups to two proteins, CheY and CheB, which controlflagellar responses and sensory adaptation, respectively (9). Phospho-CheY interacts with the switching machinery of theflagellar motors to enhance the probability of CW rotation (2, 35). Phospho-CheBcovalentlymodifiesstimulatedchemoreceptors to attenuate their signaling activity as part of a negative-feedback circuit (17). The phosphorylated forms of CheY and CheB are very short-lived, enabling the chemoreceptors to modulatetheirsteady-statelevelsinresponsetochemicalstimulibycontrollingtheautophosphorylationactivityofCheA(3). An increase in attractant level prolongs forward swimming by inhibiting CheA, thereby reducing the level of phospho-CheY. An increase in repellent level enhances the likelihood of a change in swimming direction by stimulating CheA autophosphorylation, which in turn raises phospho-CheY levels. The