Geosmin Biosynthesis in Streptomyces avermitilis. Molecular Cloning, Expression, and Mechanistic Study of the Germacradienol/Geosmin Synthase

David E. Cane, Xiaofei He, Seiji Kobayashi, Satoshi O_Bold_long.gif"mura, Haruo Ikeda

This paper is submitted in honor of the late Professor Kenneth L. Rinehart, a pioneer in the study of natural products and their biosynthesis.

Received: May 30, 2006 / Accepted: August 3, 2006
© Japan Antibiotics Research Association

Abstract Geosmin (1) is responsible for the characteristic odor of moist soil. The Gram-positive soil bacterium Streptomyces avermitilis produces geosmin (1) as well as its precursor germacradienol (3). The S. avermitilis gene SAV2163 (geoA) is extremely similar to the S. coelicolor A3(2) SCO6073 gene that encodes a germacradienol/geosmin synthase. S. avermitilis mutants with a deleted geoA were unable to produce either germacradienol (3) or geosmin (1). Biosynthesis of both compounds was restored by introducing an intact geoA gene into the mutants. Incubation of recombinant GeoA, encoded by the SAV2163 gene of S. avermitilis, with farnesyl diphosphate (2) in the presence of Mg2+ gave a mixture of (4S,7R)-germacra-1(10)E,5E-diene-11-ol (3) (66%), (7S)-germacrene D (4) (24%), geosmin (1) (8%), and a hydrocarbon, tentatively assigned the structure of octalin 5 (2%). Incubation of this germacradienol/geosmin synthase with [1,1-2H2]FPP (2a) gave geosmin-d1 (1a), as predicted. When recombinant GeoA from either S. avermitilis or S. coelicolor A3(2) was incubated with nerolidyl diphosphate (8), only the acyclic elimination products β-farnesene (10), (Z)-α-farnesene (11), and (E)-α-farnesene (12) were formed, thereby ruling out nerolidyl diphosphate as an intermediate in the conversion of farnesyl diphosphate to geosmin, germacradienol, and germacrene D.

Keywords sesquiterpene, biosynthesis, geosmin, Streptomyces avermitilis, Streptomyces coelicolor

D. E. Cane (Corresponding author), X. He: Department of Chemistry, Brown University, Box H, Providence, Rhode Island 02912-9108 USA, E-mail: David_Cane@brown.edu
S. O_long.gif"mura: The Kitasato Institute, 9-1, Shirokane 5-chome, Minato-ku, Tokyo 108-8642, Japan
S. Kobayashi, H. Ikeda: Kitasato Institute for Life Sciences, Kitasato University, 1-15-1, Kitasato, Sagamihara, Kanagawa 228-8555, Japan