Recently,several new prenyltransferases structures that contain a triose phosphate isomerase (TIM)barrel fold have been discovered,and there is considerable interest in their mechanisms of action.We recentlyreported the structures of MoeO5 containing its bound product,FPG (2-(Z,E)-farnesyl-3-phosphoglycerate)and the structures of PcrB from B.subtilis and S.aureus containing bound substrates as well as other ligandmolecules,which help define the catalytic mechanism of the TIM-barrel prenyltransferase.MoeO5 bindsFPG in a curved pocket,mainly as a result of its long λ3 loop.An FPP ionization site containing Asp41and Mg2+ is located nearby,and the results obtained herein are consistent with formation of a (Z,E)-FPPintermediate.We also find that His97 in the λ3 loop is essential for activity.And the structures of PcrBsclearly show the G1P and F(s)PP binding sites,plus,they map out much of the C35 side-chain binding pocket.The most highly conserved residues are two Tyr and one Glu in the active site,proposed to be involved in 3-OHactivation as well as H+ removal from the active site region,together with a conserved Asp,involved in Mg2+binding and diphosphate removal.In addition,we propose that Y104 is involved in chain-length regulation.