γ-Hexachlorocyclohexane (γ-HCH) is a halogenated organic insecticide, which was once used widely throughout the world, but has been prohibited in many countries.
Use of γ-HCH causes serious environmental problems because of its toxicity and long persistence in upland soil. Sphingobium japonicum UT26S utilizes γ-HCH as the sole source of carbon and energy under aerobic conditions^{1, 2}. Fifteen lin genes involved in γ-HCH degradation has been identified^{3, 4, 5}.

Sphingomonas-related strains are well known to degrade wide range of natural and xenobiotic compounds, such as chlorinated phenols, polychlorinated biphenyls and azo dyes. This suggests that the Sphingomonas strains have the ability to adapt environments probably by degrading compounds.

Genome analysis of S. japonicum UT26S revealed 5 replicons composed of two circular chromosomes, chromosome 1 (3,514,822 bp, 64.8 % G+C, 3,529 ORFs), chromosome 2 (681,892 bp, 65.9 % G+C, 589 ORFs) and three circular plasmids, pCHQ1 (190,974 bp, 63.0 % G+C, 224 ORFs), pUT1 (31,776 bp, 63.7 % G+C, 44 ORFs) and pUT2 (5,398 bp, 61.0 % G+C, 8 ORFs). Thelin genes involved in the degradation of gamma-HCH was dispersed upon 2 chromosomes and a large plasmid. Comparison of S. japonicum UT26S genome with those of other Sphingomonadaceae strains demonstrated that about a half of lin genes were located on S. japonicum UT26S specific regions, and remaining half were embedded in conserved core regions among sphingomonad strains. These results proposed the hypothesis that lin genes were gradually acquired by S. japonicum UT26S in the course of environmental adaptation.

The genome sequence of S. japonicum UT26S should provide insights for abilities and adaptation mechanisms of pesticide degrading bacteria.