## Materials and Methods
### Protein Expression and Assay  
Constructs were transformed into _Escherichia coli_ BL21-Star (Invitrogen, Carlsbad, CA, USA) or C41 OverExpress cells (Lucigen, Middleton, WI, USA). Starter cultures were inoculated into 10 mL NZY media (10 g/L NaCl, 10 g/L casein, 5 g/L yeast extract, 1 g/L MgSO4 (anhydrous), pH 7.0) with 50 \(\upmu\)g/mL carbenicillin and incubated at 200 rpm and 18 \({}^{\circ}\)C for 3 days. Starter cultures (5 mL) were used to inoculate 1000 mL fresh NZY media, also with 50 \(\upmu\)g/mL carbenicillin, and after reaching an OD\({}_{600}\) of 0.6 they were induced with 1 Mm Isopropyl \(\upbeta\)-d-1-thiogalactopyranoside (IPTG) and incubated under continuous shaking at 200 rpm at 18 \({}^{\circ}\)C for 12 hours. Cells were harvested by centrifugation at 5000\(\times\)\(g\) for 10 min. The cell pellet was re-suspended in 5 mL 25 mM 3-(N-morpholino)-2-hydroxypropanesulfonic acid buffer (MOPSO), pH 7.2, 10 mM MgCl\({}_{2}\), 10% (\(\
u/v\)) glycerol with 10 mM imidazole, and homogenized using an EmulsiFlex C-5 (Avestin, Canada). The homogenized suspension was centrifuged at 16,000\(\times\)\(g\) for 60 min. The supernatant was passed over 1 mL Ni-NTA agarose (Qiagen, Hilden, Germany) washed with 5 mL buffer containing 10 mM imidazole and an additional 5 mL with 50 mM imidazole. Proteins were eluted with 2 mL buffer containing 250 mM imidazole.  
Enzyme assays were carried out with 300 \(\upmu\)g purified protein in 2 mL 25 mM MOPSO, pH 7.2, 10 mM MgCl2, 10% (\(\
u/v\)) glycerol with the addition of 100 \(\upmu\)M DMAPP and 10 or 100 \(\upmu\)M IPP, the assays were incubated for either 30 minutes or 24 hours at 30 \({}^{\circ}\)C before addition of 200 units of calf intestinal alkaline phosphatase (Promega, Madison, WI) and the supplied buffer. The assay was dephosphorylated for 24 hours at 30 \({}^{\circ}\)C and extracted three times with 2 mL pentane. The organic phase was combined and concentrated under a gentle stream of N\({}_{2}\) until a volume of 200 \(\upmu\)L solvent remained. The concentrated organic extracts (1 \(\upmu\)L) were injected into 3900 Saturn GC (Varian, Palo Alto, CA) with an injector temperature of 250 \({}^{\circ}\)C coupled to a Saturn 2100T ion trap mass spectrometer detector for product identification. Separation was achieved using a HP-5 MS column (30 m \(\times\) 250 \(\upmu\)m \(\times\) 0.25 \(\upmu\)m) (Agilent, Santa Clara, CA, USA). The chromatographic program was as follows: 50 \({}^{\circ}\)C for 3 min, increasing to 300 \({}^{\circ}\)C by 15 \({}^{\circ}\)C/min and held at 300 \({}^{\circ}\)C 3 min. Electrospray ionization scanning from 60-650 \(m/z\) was used to obtain product spectra (Figure S3). Quantification of products was completed using a Shimadzu GC02014 with flame ionization detection (GC-FID) over an SH-Rxi-5 ms column (15 m \(\times\) 250 \(\upmu\)m \(\times\) 0.25 \(\upmu\)m; Shimadzu, Kyoto, Japan) using the previously described method. Products were confirmed by comparison, of both retention time and mass spectra (Figure S3), to isoprenoid diphosphate authentic standards dephosphorylated as above.  
Supplementary Materials: Supplementary materials are available on line.  
[PERSON] and [PERSON] conceived the research and wrote the manuscript; [PERSON], [PERSON], [PERSON] and [PERSON] performed research, analyzed data and revised the manuscript.  
**Funding:** This work was supported by a grant from the NIH (GM076324) to [PERSON] and a postdoctoral fellowship to [PERSON] from the Deutsche Forschungsgemeinschaft (DFG) NA 1261/1-2.  
**Acknowledgments:** The authors thank [PERSON] for assistance with constructing mutants and expression vectors.  
**Conflicts of Interest:** The authors declare no conflict of interest.