Identification and Characterization of a Novel Nitroreductase Transforming the Herbicide Mesotrione in Metabacillus sp. JX24

Mesotrione, a commonly used 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor herbicide, threatens agro-ecosystem sustainability and nontarget organisms. Microbes play a significant role in the biodegradation of mesotrione. However, an understanding of the molecular mechanisms involved in mesotrione degradation is still quite limited. Here, a bacterial strain Metabacillus sp. JX24 capable of degrading mesotrione was isolated. The strain could effectively degrade about 98% of 100 μM mesotrione within 12 h. It was revealed that mesotrione was degraded through successive reduction of its nitro group to generate 2-amino-4-methylsulfonylbenzoic acid (AMBA). A gene, mnrA, encoding a novel nitroreductase MnrA responsible for the transformation of mesotrione, was cloned from strain JX24. MnrA shared low sequence identities (<20%) with the reported mesotrione nitroreductases. The purified MnrA catalyzed the nitro reduction of mesotrione with NADH or NADPH as cofactors in vitro. The K_m and k_cat/K_m values of MnrA for mesotrione were 22.0 μM and 0.57 μM^–1 min^–1, respectively. The detoxification assay indicated that AMBA, the product of MnrA-mediated mesotrione reduction, did not inhibit HPPD activity, suggesting that MnrA confers a detoxifying action on mesotrione. Our work provides a novel enzymatic resource for the elimination of mesotrione residues in the environment.