The TetA(B) tetracycline/H+ antiporter. Model is based on many publications by several research groups and is from Sapunaric et al. 2005 in D.G. White et al (eds), Frontiers in Antimicrobial Resistance: a Tribute to Stuart B. Levy, ASM Press. Transmembrane a-helices (TM) are indicated by gray rectangles numbered TM1-TM12. Circles -- residues of class B that affect substrate specificity or affinity and so are possibly involved in substrate binding. Bracket interdomain loop region corresponding to site in class A possibly involved in substrate binding. Squares--amino acids critical for tetracycline efflux. Triangles -- amino acids possibly involved in local conformation change. Arrow -- site where tetracycline/Fe++ substrate cleaves the protein backbone by a Fenton reaction. Scissors -- position where a truncation permits good activity. Horizontal lines -- permeability barrier for hydrophilic molecules. Circled star -- location in TetA of class C of Ser202 involved in transport activity. Other stars -- sites in class C of secondary suppressor mutations of the inactivating Ser202Phe substitution, that may represent residues that interact directly with Ser202.