Adaptive evolution of Sphingopyxis sp. MC4 conferred degradation potential for persistent β- and δ-Hexachlorocyclohexane (HCH) isomers

Hexachlorocyclohexane (HCH), an organochlorine pesticide imposes several harmful impacts on the ecosystem. 13- and 6-isomers of HCH are highly toxic, persistent, and recalcitrant to biodegradation, slow and incomplete degradation of 13- and 6-isomers have been reported in a few strains. We have isolated a strain designated as Sphingopyxis strain MC4 that can tolerate and degrade high concentrations of a-, 13-, y- and 6-HCH isomers. To date, no other Sphingopyxis strain has been reported to degrade 13- and 6-isomers. To understand the underlying genetic makeup contributing to adaptations, the whole genome of strain MC4 was sequenced. Comparative genome analysis showed that strain MC4 harbors the complete pathway (lin genes) required for HCH degradation. Genetic footprints such as presence of lin genes on genomic islands, IS6100 elements in close proximity of lin genes, and synteny in lin flanking regions with other strains reflects the horizontal gene transfer in strain MC4. Positive selection and HGT drive the adaptive evolution of strain MC4 under the pressure of HCH contamination that it experienced in its surrounding niche. In silico analyses showed efficient binding of 13- and 6-isomers with enzymes leading to rapid degradation that need further validation by cloning and biochemical experiments.