A single amino acid substitution alters ligand specificity of the chemoreceptor CtaB in <i>Pseudomonas protegens</i> CHA0

Chemotaxis enables bacteria to move toward favorable compounds via chemoreceptors. CtaA and CtaB are amino acid chemoreceptors in Pseudomonas protegens CHA0, and their ligand-binding domains show high sequence similarity. However, CtaA exhibits broad specificity, recognizing all 20 naturally occurring l-amino acids as ligands, whereas CtaB senses only four. This study aimed to investigate residues determining ligand specificity using site-directed mutagenesis and in silico analyses. Chemotaxis assays with heterologously complemented strains revealed that the D146A mutation in CtaA completely eliminated its ability to recognize ligands, whereas the A144D mutation in CtaB, corresponding to D146 in CtaA, enabled it to recognize new ligands while abolishing its original specificity. Hence, the residue at position 144 is a key determinant of CtaB specificity. Structural and docking analyses further suggested that other residues, including G99/F97 and I111/Q109 (CtaA and CtaB, respectively), may also contribute to differences in ligand specificity between CtaA and CtaB.