ScGS: Streptomyces coelicolor geosmin synthase
Geosmin synthase from Streptomyces coelicolor A3(2) (ScGS) is a single 726-amino acid protein which catalyzes the Mg2+ dependent conversion of farnesyl diphosphate to a mixture including geosmin. ScGS is a bifunctional enzyme whose N-terminal domain catalyzes the cyclization of FPP to form germacradienol, while C-terminal domain then converts this sesquiterpenoid product to geosmin.
Usage and Biology
The ScGS is a bifunctional sesquiterpene cyclase, with the presence of Mg2+, the N-terminal half of this protein catalyzes the ionization and cyclization of farnesyl diphosphate to form germacradienol and inorganic pyrophosphate(PPi). Then the C-terminal domain, highly homologous with the former, catalyzes the protonation, cyclization, and fragmentation of germacradienol to form geosmin and acetone.
Sequence and Features
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To obtain ScGS, pET-28a(+)-ScGS(with His-tag) was transferred into E.coli BL21(DE3), and the cells were inoculated in 25 mL cultures of LB medium with 10 μg/mL kanamycin. These cultures were grown at 37℃ with 250 rpm shaking until the OD600 reached 0.5-0.8, then 0.3 mM isopropyl β-D-1-thiogalactopyranoside(IPTG) were added, following by an overnight cultivation at 16℃ with 250 rpm shaking to induce protein expression. The washed and harvested cells were resuspended with a Binding Buffer, and then the cells were lysed by ultrasonication. Purification was performed according to the protocol of Ni-NTA SefinoseTM Resin (Sangon Biotech, Shanghai, China). As it shows in the following figure(Figure 1.), the existence of ScGS in our chasis was proved by SDS-PAGE analysis.
In order to identify the synthesis of geosmin, engineered bacteria in TB medium containing 5% glycerol were first induced ScGS expression with 0.7mM IPTG when OD600 reached about 0.7, following by an overnight culture at 18℃ and continuing cultivation for next 72h at 25℃. From this way we could smell a strong and unusual odor from the culture comparing to the control.
For further demonstration, we prepared the sample via headspace liguid-phase microextraction(HS-LPME) and a gas chromatography-mass spectrometry(GC-MS) test was conducted. The results given by GC-MS fairly shows the existence of geosmin in our culture(Figure 2.), thus proves the feasibility of the part.
 Harris G G, Lombardi P M, Pemberton T A, et al. Structural Studies of Geosmin Synthase, a Bifunctional Sesquiterpene Synthase with αα Domain Architecture That Catalyzes a Unique Cyclization–Fragmentation Reaction Sequence[J]. Biochemistry, 2015, 54(48): 7142-7155.
 Xie Y, He J, Huang J, et al. Determination of 2-methylisoborneol and geosmin produced by Streptomyces sp. and Anabaena PCC7120[J]. Journal of agricultural and food chemistry, 2007, 55(17): 6823-6828.