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γ-Hexachlorocyclohexane (γ-HCH) -degrading strain, Sphingobium sp. TA15, was newly isolated from an experimental field soil from which the archetypal γ -HCH-degrading strain, S. japonicum UT26, was isolated previously. Comparison of the complete genome sequences of these 2 strains revealed that TA15 shares the same basic genome backbone with UT26, but also has the variable regions that are presumed to have changed either from UT26 or from a putative common ancestor. Organization and localization of lin genes of TA15 were different from those of UT26. It was inferred that transposition of IS 6100 had played a crucial role in these genome rearrangements. The accumulation of toxic dead-end products in TA15 was lower than in UT26, suggesting that TA15 utilizes γ-HCH more effectively than UT26. These results suggested that genome evolution related to the γ-HCH metabolic function in the soil microbial population is ongoing. [ABSTRACT FROM AUTHOR]