Abstract
A 6-chloronicotinic acid mineralizing bacterium was isolated from enrichment cultures originating from imidacloprid-contaminated soil samples. This Bradyrhizobiaceae, designated strain SG-6C, hydrolytically dechlorinated 6-chloronicotinic acid to 6-hydroxynicotinic acid, which was then further metabolised via the nicotinic acid pathway. This metabolic pathway was confirmed by growth and resting cell assays using HPLC and LC-MS studies. A candidate for the gene encoding the initial dechlorination step, named cch2 (for 6-chloronicotinic acid chlorohydrolase), was identified using genome sequencing and its function was confirmed using resting cell assays on E. coli heterologously expressing this gene. The 464 amino acid enzyme was found to be a member of the metal dependent hydrolase superfamily with similarities to the TRZ/ATZ family of chlorohydrolases. We also provide evidence that cch2 was mobilized into this bacterium by an Integrative and Conjugative Element (ICE) that feeds 6-hydroxynicotinic acid into the existing nicotinic acid mineralization pathway.
Highlights
Over the last two decades, neonicotinoids have risen to become one of the most widely used classes of insecticides against a broad spectrum of crop and domestic pests [1,2]
Studies on the fate of 6-chloronicotinoic acid (6-CNA) in mice and spinach have established that it is removed from the system through various conjugated metabolites, there are no reports on the fate of 6-CNA in the environment [7]. 6-CNA has been found to accumulate as a major metabolite (0.5 to 1 ppm) in soils after 2 months of IMI application (7.2 ppm) [12]
We provide biochemical and genomic evidence that 6-CNA is degraded via a pre-existing nicotinic acid (NA) catabolic pathway in this bacterium and that the gene encoding the 6-CNA dechlorinating gene-enzyme system has been acquired through horizontal gene transfer of an Integrative and Conjugating Element (ICE) [13]
Summary
Over the last two decades, neonicotinoids have risen to become one of the most widely used classes of insecticides against a broad spectrum of crop and domestic pests [1,2]. We provide biochemical and genomic evidence that 6-CNA is degraded via a pre-existing nicotinic acid (NA) catabolic pathway in this bacterium and that the gene encoding the 6-CNA dechlorinating gene-enzyme system has been acquired through horizontal gene transfer of an Integrative and Conjugating Element (ICE) [13]. As the sole carbon source, a 1% v/v seed culture of strain SG-6C completely degraded the 6-CNA within 152 h (Figure 2).
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