THE SMALLER OF 2 OVERLAPPING CHEA GENE-PRODUCTS IS NOT ESSENTIAL FOR CHEMOTAXIS IN ESCHERICHIA-COLI

The cheA locus of Escherichia coli encodes two similar proteins, CheA(L) (654 amino acids) and CheA(S) (557 amino acids), which are made by initiating translation from different in-frame start sites [start(L) and start(S)]. CheA(L) plays an essential role in chemotactic signaling. It autophosphorylates at a histidine residue (His-48) and then donates this phosphate to response regulator proteins that modulate flagellar rotation and sensory adaptation, CheA(S) lacks the first 97 amino acids of CheA(L), including the phosphorylation site at His-48. Although it is unable to autophosphorylate, CheA(S) can form heterodimers with mutant CheA(L) subunits to restore kinase function and chemoreceptor control of autophosphorylation activity, To determine whether these or other activities of CheA(S) are important for chemotaxis, we constructed cheA lesions that abrogated CheA(S) expression, Mutants in which the CheA(S) start codon was changed from methionine to isoleucine (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 CheA(S) from a specialized transducing phage, indicating that the lesions in CheA(L)-not the lack of CheA(S)-were responsible for the reduced chemotactic ability, In other respects, the behavior of the M98I mutant was essentially normal, Its flagellar rotation pattern was indistinguishable from wild type, and it exhibited wild-type detection thresholds and peak positions in capillary chemotaxis assays, The lack of any substantive defect in this start(S) mutant argues that CheA(S) makes a negligible contribution to chemotactic ability in the laboratory. Whether it has functional significance in other settings remains to be seen,