Tow-domain protein where the N-terminal domain catalyzes the cyclization of farnesyl diphosphate (FPP) to a 85:15 mixture of the sesquiterpene alcohol germacradienol and the sesquiterpene hydrocarbon germacrene D. The C-terminal domain partially converts the germacradienol formed into geosmin, the characteristic odoriferous ('earthy aroma') constituent of Streptomyces species. (2E,6E)-farnesyl diphosphate + H2O = (1E,4S,5E,7R)-germacra-1(10),5-dien-11-ol + diphosphate (1E,4S,5E,7R)-germacra-1(10),5-dien-11-ol + H2O = (-)-geosmin + acetone (2E,6E)-farnesyl diphosphate = (-)-germacrene D + diphosphate Magnesium. Fe(2+) or Cu(2+) ions are very less efficient as cofactors. Secondary metabolite biosynthesis; geosmin biosynthesis. Sesquiterpene biosynthesis; germacradienol biosynthesis; germacradienol from farnesyl diphosphate: step 1/1. Sesquiterpene biosynthesis; germacrene D biosynthesis; germacrene D from farnesyl diphosphate: step 1/1. Consists of 2 homologous sesquiterpene synthase domains. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). The earthy odorant geosmin is also responsible for the 'off-flavor' of contaminated drinking water, wines, and other foodstuffs. Belongs to the terpene synthase family. It is uncertain whether the initial Met is cleaved or not. The earth's perfume - Issue 35 of June 2003