Appearance and evolution of γ-hexachlorocyclohexane-degrading bacteria

α-Proteobacterial strains belonging to the so-called sphingomonads group degrade various highly recalcitrant compounds, including xenobiotics, but generally each strain degrades only a limited set of compounds, suggesting that sphingomonads tend to be specialists for the degradation of extremely recalcitrant compounds. In this chapter, the appearance and evolution of γ-hexachlorocyclohexane (γ-HCH/γ-BHC/lindane)-degrading bacteria belonging to sphingomonads will be discussed on the basis of the structure and function of their genomes and mobile genetic elements. γ-HCH is a typical, completely man-made chlorinated pesticide that has caused serious environmental problems due to its toxicity and long persistence in upland soils. The genome sequence of an archetypal γ-HCH-degrading strain, Sphingobium japonicum UT26, and its comparison with those of other closely related γ-HCH-degrading and non-γ-HCH-degrading sphingomonad strains revealed that (1) these γ-HCH-degraders appeared independently and in parallel at geographically different areas by recruiting the specific lin genes into strains having the core functions of sphingomonads; (2) various sphingomonad-specific plasmids and the insertion sequence IS6100 play important roles in the recruitment and dissemination of the specific lin genes; and (3) transposition of IS6100 causes genome rearrangements including deletion and inversion of DNA sequences and fusion and resolution of replicons.