| The volatile metabolite geosmin is a degraded sesquiterpene that is responsible for the characteristic smell of moist soil. Geosmin is produced by a wide range of microorganisms, including most species of Streptomyces and a variety of other actinomycetes, myxobacteria and cyanobacteria, as well as certain fungi and selected higher plants such as liverworts and beets. In S. coelicolor A3(2) a single 726-amino acid protein, encoded by the 2181-bp SCO6073 gene (cyc2), catalyzes the Mg 2+-dependent cyclization of farnesyl diphosphate (FPP), the universal acyclic precursor of all sesquiterpenes, to a mixture of geosmin, germacradienol, germacrene D and small amount of 8,10-dimethyl-1-octalin. The S. coelicolor germacradienol-geosmin synthase is in fact a bifunctional enzyme in which the N-terminal domain catalyzes the cyclization of FPP to an 85:15 mixture of germacradienol and germacrene D, accompanied by traces of the octalin, while the C-terminal domain catalyzes the Mg2+-dependent cyclization--fragmentation of germacradienol to geosmin, with release of the 2-propanol side chain as acetone. Site-directed mutagenesis experiments have confirmed that the N- and C-terminal domains each harbor catalytically independent active sites.;We also demonstrated geosmin biosynthesis in a model cyanobacterium, Nostoc punctiforme PCC 73102, which utilizes a single enzyme to catalyze the cyclization of FPP to geosmin. Using this information, we developed a PCR-based assay that allows the rapid detection of geosmin-producing cyanobacteria. This test may be utilized to confirm and track the emergence of taste and odor-producing cyanobacteria in any given water body and thus can be used as an early warning system by managers of drinking water bodies that may suffer from adverse taste and odor episodes.;Pentalenolactone is a sesquiterpenoid antibiotic isolated from a variety of Streptomyces species. The ptlE gene from the pentalenolactone biosynthetic gene cluster of S. avermitilis encodes a 594 aa flavin-dependent monooxygenase, which catalyzes the Baeyer-Villiger oxidation of 1-deoxy-11-oxopentalenic acid to an unexpected product of neopentalenolactone D. The structure of this compound has been confirmed by GC-MS and NMR. |
