Fig. 1.

Three transmembrane signaling systems found in enteric bacteria. Extracellular inputs (Top, cartoon color scale from dark N terminus to light C terminus) and cytoplasmic outputs (Bottom) are depicted for three representative types of bacterial signaling systems. (A) Two-component PhoQ (Protein Data Bank (PDB): 1yax) periplasmic input sensor domain (green) is attached through a TMH to an intracellular histidine kinase that signals to a second component, the PhoP output response regulator. (B) One-component CadC (PDB: 3ly7) periplasmic domain (green) is attached through a TMH to the output HTH domain. (C) Co-component VtrA/VtrC (PDB: 5kew) periplasmic input domains (green/blue), with VtrC attached through a TMH to the output HTH domain.