FIG 6.

Model for pH sensing mechanism in B. subtilis. (A) At low pH, two ionizable residues (black-outlined green circle) on TlpB are in their protonated state and form hydrogen bonds with two adjacent residues (white circle). Deprotonation of these residues upon pH increase disrupts the local structure due to decreased hydrogen bonding and induces signaling. (B) At high pH, the key histidine residue (lower white circle) within the pH-sensing region of McpA is in neutral state and forms hydrogen bonds with adjacent residues (upper white circle). As pH decreases, the histidine residue becomes protonated (black-outlined green circle), leading to loss of hydrogen bonding. This disrupts the local structure and induces signaling.