Efficient biodegradation of dihalogenated benzonitrile herbicides by recombinant Escherichia coli harboring nitrile hydratase-amidase pathway

Abstract

Worldwide use of benzonitrile herbicides has caused a contamination hazard for groundwater, and their removal has attracted increasing attention. Microbial degradation has been considered as a major route of removing toxic nitriles from the environment. However, the process is very inefficient for degrading benzonitrile herbicides by natural microbes. In this study, a recombinant microbial cell was constructed to degrade benzonitrile herbicides by co-expression of nitrile hydratase and amidase in Escherichia coli. Both enzymes were functionally over-expressed in the cytoplasm of E. coli. The NHase activities of the cell reactor on dichlobenil and ioxynil were 15.4 and 21.3U/mg dry cell weigh (DCW), respectively. And the amidase activities on 2,6-dichlorobenzamide and 3,5-iodo-4-hydroxybenzamide were 8.3 and 13.6U/mg DCW, respectively. Furthermore, the degradation of dichlobenil and ioxynil was investigated using the recombinant cell reactor. The degradation process suggested that dichlobenil and ioxynil were degraded to corresponding carboxylic acids via nitrile hydratase-amidase pathway, and the intermediate amides (2,6-dichlorobenzamide and 3,5-iodo-4-hydroxybenzamide) did not accumulate in the reaction mixture. The degradation rates of dichlobenil and ioxynil were 43 and 185mg/g DCW/h, respectively. The recombinant E. coli cell reactor was observed to be promising catalysts for the bioremediation of wastewater containing benzonitrile herbicides.