Cloning of a novel 6-chloronicotinic acid chlorohydrolase from the newly isolated 6-chloronicotinic acid mineralizing Bradyrhizobiaceae strain SG-6C.

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.