A plasmid, pTA163, in Escherichia coli contained an approximately 34-kb gene fragment from Pseudomonas putida JYR-1 that included the genes responsible for the metabolism of trans-anethole to protocatechuic acid. Three Tn5-disrupted open reading frame 10 (ORF 10) mutants of plasmid pTA163 lost their abilities to catalyze trans-anethole. Heterologously expressed ORF 10 (1,047 nucleotides [nt]) under a T7 promoter in E. coli catalyzed oxidative cleavage of a propenyl group of trans-anethole to an aldehyde group, resulting in the production of para-anisaldehyde, and this gene was designated tao (trans-anethole oxygenase). The deduced amino acid sequence of TAO had the highest identity (34%) to a hypothetical protein of Agrobacterium vitis S4 and likely contained a flavin-binding site. Preferred incorporation of an oxygen molecule from water into p-anisaldehyde using (18)O-labeling experiments indicated stereo preference of TAO for hydrolysis of the epoxide group. Interestingly, unlike the narrow substrate range of isoeugenol monooxygenase from Pseudomonas putida IE27 and Pseudomonas nitroreducens Jin1, TAO from P. putida JYR-1 catalyzed isoeugenol, O-methyl isoeugenol, and isosafrole, all of which contain the 2-propenyl functional group on the aromatic ring structure. Addition of NAD(P)H to the ultrafiltered cell extracts of E. coli (pTA163) increased the activity of TAO. Due to the relaxed substrate range of TAO, it may be utilized for the production of various fragrance compounds from plant phenylpropanoids in the future.
Bacterial strain JYR-1, which utilizes high concentrations (up to 100 mM) of trans-anethole as the sole source of carbon and energy, was isolated from soil. It grew to OD600nm = 2.6 with a doubling time of 8 h when grown on 20 mM trans-anethole. Strain JYR-1 was identified as Pseudomonas putida based on the partial gene sequence of its 16S rDNA. Elution profiles of culture extracts were examined by high-performance liquid chromatography and showed that four metabolites were produced from the bacterial culture containing trans-anethole that were not detected in control experiments. LC-MS analysis showed molecular weights of 138.2, 164.5, 164.3, and 152.3. The metabolites with molecular weights at 152.3 and 138.2 were confirmed to be p-anisic acid and p-hydroxybenzoic acid, respectively, when compared with HPLC retention times and molecular weights of authentic compounds. The metabolites with molecular weights at 164.5 and 164.3 were further analyzed by NMR and were proved to be stereoisomer syn- and anti-anethole epoxides. Therefore, strain JYR-1 most likely initiates the metabolism of trans-anethole through the formation of epoxides on the propene group of the compound. Keywords: trans-Anethole; Pseudomonas; flavor; phenylpropanoid; biotransformation
A gene encoding p-anisaldehyde dehydrogenase (PAADH), which catalyzes the oxidation of p-anisaldehyde to p-anisic acid, was identified to be clustered with the trans-anethole oxygenase (tao) gene in Pseudomonas putida JYR-1. Heterologously expressed PAADH in Escherichia coli catalyzed the oxidation of vanillin, veratraldehyde, and piperonal to the corresponding aromatic acids vanillic acid, veratric acid, and piperonylic acid, respectively. Coexpression of trans-anethole oxygenase (TAO) and PAADH in E. coli also resulted in the successful transformation of trans-anethole, isoeugenol, O-methyl isoeugenol, and isosafrole to p-anisic acid, vanillic acid, veratric acid, and piperonylic acid, respectively, which are compounds found in plants as secondary metabolites. Because of the relaxed substrate specificity and high transformation rates by coexpressed TAO and PAADH in E. coli , the engineered strain has potential to be applied in the fragrance industry.
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