Four chemoreceptors govern bidirectional pH taxis in Bacillus subtilis

We investigated pH taxis in Bacillus subtilis. This bacterium was found to perform bidirectional taxis in response to external pH gradients, enabling it to preferentially migrate to neutral environments. We next investigated the chemoreceptors involved in sensing pH gradients. We found that four chemoreceptors are involved in sensing pH: McpA and TlpA for sensing acidic environments and McpB and TlpB for alkaline ones. In addition, TlpA was found to also weakly sense alkaline environments. By analyzing chimeras between McpA and TlpB, the principal acid and base-sensing chemoreceptors, we identified four critical amino-acid residues - Thr199, Gln200, His273, and Glu274 on McpA and Lys199, Glu200, Gln273, and Asp274 on TlpB - involved in sensing pH. Swapping these four residues between McpA and TlpB converted the former into a base receptor and the latter into an acid receptor. Based on the results, we propose that disruption of hydrogen bonding between the adjacent residues upon pH changes induces signaling. Collectively, our results further our understanding of chemotaxis in B. subtilis and provide a new model for pH sensing in bacteria.\n\nIMPORTANCEMany bacteria can sense the pH in their environment and then use this information to direct their movement towards more favorable locations. In this study, we investigated the pH sensing mechanism in Bacillus subtilis. This bacterium preferentially migrates to neutral environments. It employs four chemoreceptors to sense pH. Two are involved in sensing acidic environments and two are involved in sensing alkaline ones. To identify the mechanism for pH sensing, we constructed receptor chimeras of acid and base sensing chemoreceptors. By analyzing the response of these chimeric receptors, we were able to identify four critical amino-acid residues involved in pH sensing and propose a model for the pH sensing mechanism in B. subtilis.