Undecaprenyl pyrophosphate synthase (UPPs) catalyzes eight consecutive condensation reactions of farnesyl pyrophosphate (FPP) with isopentenyl pyrophosphate (IPP) to form a 55-carbon long-chain product. We previously reported the crystal structure of the apo-enzyme from Escherichia coli and the structure of UPPs in complex with sulfate ions (resembling pyrophosphate of substrate), Mg 2+ , and two Triton molecules (product-like). In the present study, FPP substrate was soaked into the UPPs crystals, and the complex structure was solved. Based on the crystal structure, the pyrophosphate head group of FPP is bound to the backbone NHs of Gly29 and Arg30 as well as the side chains of Asn28, Arg30, and Arg39 through hydrogen bonds. His43 is close to the C2 carbon of FPP and may stabilize the farnesyl cation intermediate during catalysis. The hydrocarbon moiety of FPP is bound with hydrophobic amino acids including Leu85, Leu88, and Phe89, located on the ␣3 helix. The binding mode of FPP in cis-type UPPs is apparently different from that of trans-type and many other prenyltransferases which utilize Asp-rich motifs for substrate binding via Mg 2+ . The new structure provides a plausible mechanism for the catalysis of UPPs.Keywords: prenyltransferase; farnesyl pyrophosphate; isopentenyl pyrophosphate; crystal structure; substrate binding; metal ion (Sato et al. 1999;Chang et al. 2001). Rubber prenyltransferase synthesizes a huge polymer containing thousands of IPP units (Cornish 2001). In contrast, trans-prenyltransferases, which generate trans-double bonds in IPP condensation, synthesize shorter chain-length products, C 15 -C 50 (Chen et al. 1994; Wang and Ohuma 2000). Abbreviations: UPPs, undecaprenyl pyrophosphate synthase; IPP, isopentenyl pyrophosphate; FPP, farnesyl pyrophosphate; UPP, undecaprenyl pyrophosphate; FPPs, farnesyl pyrophosphate synthase; GPP, geranyl pyrophosphate; TLC, thin-layer chromatography; NiNTA, nickel nitrilo-triacetic acid; HEPES, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; EDTA, ethylenediaminetetraacetic acid; FTase, farnesyltransferase.Article and publication are at http://www.proteinscience.org/cgi