Long Zhang,Man Song,Zhenbo Mao,Yuan Liu,Feng Li,Jiandong Jiang,Kai Chen
Dichlorprop [( R , S )-2-(2,4-dichlorophenoxy)propanoic acid], a chiral herbicide belonging to phenoxyalkanoic acid herbicides, is extensively used around the world, and is frequently detected in various habitats. Although ( S )-dichlorprop has no herbicidal activity, its toxicity and ecological risk are higher than ( R )-dichlorprop. In our previous studies, the isolated strain Sphingopyxis sp. DBS4 was confirmed to mineralize both ( R )-dichlorprop and ( S )-dichlorprop, but the key genes responsible for the initial degradation of ( R )-/( S )-dichlorprop were still unclear. Here, based on genome sequencing and enzyme purification , a new dioxygenase gene, spoA , responsible for the initial transformation of ( S )-dichlorprop to 2,4-dichlorophenol and pyruvate was cloned from the rac -dichlorprop-utilizing strain of Sphingopyxis sp. DBS4. SpoA, an α -ketoglutarate-dependent dioxygenase, exhibited low amino acid sequence identity to the reported ( S )-dichlorprop-transforming dioxygenase SdpA (34.8%), and showed enantioselective activity toward ( S )-enantiomers of dichlorprop and mecoprop, but no activity toward the corresponding ( R )-enantiomers. The K m value of SpoA for ( S )-dichlorprop was 134.3 μM, and the k cat / K m was determined to be 7.5 × 10 −2 μM −1 s −1 . Molecular docking and site-directed mutagenesis showed that two residues of SdpA, Lys101 and Asp119, are the key sites for the enantioselective transformation of ( S )-dichlorprop. Our study identifies a new dioxygenase for the enantioselective transformation of ( S )-dichlorprop and deepens our understanding of the microbial catabolism of chiral phenoxyalkanoic acid herbicides.
Long Zhang,Ping Hang,Xi-Yi Zhou,Wen-Jing Qiao,Jian-Dong Jiang
Dichlorprop [(RS)-2-(2,4-dichlorophenoxy)propanoic acid; DCPP], an important phenoxyalkanoic acid herbicide (PAAH), is extensively used in the form of racemic mixtures (Rac-DCPP), and the environmental fates of both DCPP enantiomers [(R)-DCPP and (S)-DCPP] mediated by microorganisms are of great concern. In this study, a bacterial strain Sphingopyxis sp. DBS4 was isolated from contaminated soil and was capable of utilizing both (R)-DCPP and (S)-DCPP as the sole carbon source for growth. Strain DBS4 preferentially catabolized (S)-DCPP as compared to (R)-DCPP. The optimal conditions for Rac-DCPP degradation by strain DBS4 were 30 °C and pH 7.0. In addition to Rac-DCPP, other PAAHs such as (RS)-2-(4-chloro-2-methylphenoxy)propanoic acid, 2,4-dichlorophenoxyacetic acid, 4-chloro-2-methylphenoxyacetic acid, and 2,4-dichlorophenoxyacetic acid butyl ester could also be catabolized by strain DBS4. Bioremediation of Rac-DCPP-contaminated soil by inoculation of strain DBS4 exhibited an effective removal of both (R)-DCPP and (S)-DCPP from the soil. Due to its broad substrate spectrum, strain DBS4 showed great potential in the bioremediation of PAAH-contaminated sites.
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