Hexachlorocyclohexane (HCH), an organochlorine pesticide imposes several harmful impacts on the ecosystem. β- and δ-isomers of HCH are highly toxic, persistent, and recalcitrant to biodegradation, slow and incomplete degradation of β- and δ- 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 α-, β-, γ- and δ-HCH isomers. To date, no other Sphingopyxis strain has been reported to degrade β- and δ-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 β- and δ-isomers with enzymes leading to rapid degradation that need further validation by cloning and biochemical experiments.

六氯环己烷 (HCH) 是一种有机氯农药，对生态系统造成多种有害影响。六氯环己烷的β-和δ-异构体具有剧毒、持久性和难生物降解性，据报道在少数菌株中β-和δ-异构体降解缓慢且不完全。我们分离出了一种菌株，命名为鞘氨醇菌株 MC4，它可以耐受并降解高浓度的 α-、β-、γ- 和δ -HCH 异构体。迄今为止，尚无其他鞘氨醇菌株可降解 β- 和δ-异构体的报道。为了了解影响适应的潜在基因组成，对 MC4 菌株的整个基因组进行了测序。比较基因组分析表明，菌株 MC4 具有六氯环己烷降解所需的完整途径（ lin基因）。基因足迹，例如基因组岛上存在lin基因、紧邻lin基因的 IS 6100元件以及lin侧翼区域与其他菌株的同线性，反映了菌株 MC4 中的水平基因转移。正向选择和 HGT 推动了 MC4 菌株在其周围生态位所经历的六氯环己烷污染压力下的适应性进化。计算机分析显示β-和δ-异构体与酶的有效结合导致快速降解，需要通过克隆和生化实验进一步验证。