Structures of CrtM and SQS. a) S. aureus CrtM (green) with FSPP, Mg2+ superimposed on human SQS (orange): essential Asp residues and Mg2+ colored as in Fig. 1. b) Active site region in CrtM + FSPP (green, yellow), Mg2+. c) PSPP, Mg2+ (all in cyan) bound to CrtM, superimposed on FSPP/Mg2+ structure (in green/yellow/blue). d) Dehydrosqualene product (pink) bound to CrtM, superimposed on PSPP structure (cyan). S1 = allylic binding site; S2 = homoallylic binding site.