Aerobic degradation of lindane (γ-hexachlorocyclohexane) in bacteria and its biochemical and molecular basis

γ-Hexachlorocyclohexane (γ-HCH, also called γ-BHC and lindane) is a halogenated organic insecticide that causes serious environmental problems. The aerobic degradation pathway of γ-HCH was extensively revealed in bacterial strain Sphingobium japonicum (formerly Sphingomonas paucimobilis) UT26. γ-HCH is transformed to 2,5-dichlorohydroquinone through sequential reactions catalyzed by LinA, LinB, and LinC, and then 2,5-dichlorohydroquinone is further metabolized by LinD, LinE, LinF, LinGH, and LinJ to succinyl-CoA and acetyl-CoA, which are metabolized in the citrate/tricarboxylic acid cycle. In addition to these catalytic enzymes, a putative ABC-type transporter system encoded by linKLMN is also essential for the γ-HCH utilization in UT26. Preliminary examination of the complete genome sequence of UT26 clearly demonstrated that lin genes for the γ-HCH utilization are dispersed on three large circular replicons with sizes of 3.5 Mb, 682 kb, and 191 kb. Nearly identical lin genes were also found in other HCH-degrading bacterial strains, and it has been suggested that the distribution of lin genes is mainly mediated by insertion sequence IS6100 and plasmids. Recently, it was revealed that two dehalogenases, LinA and LinB, have variants with small number of amino acid differences, and they showed dramatic functional differences for the degradation of HCH isomers, indicating these enzymes are still evolving at high speed.