Study of genome of pesticide-degrading bacteria as a representative of environmental bacterium

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Mechanism for adaptation of environmental bacteria is of great interest. Sphingobium japonicum (formerly Sphingomonas paucimobilis) UT26 utilizes the important insecticide gamma-hexachlorocyclohexane (HCH) as a sole source of carbon and energy. In this study it was demonstrated that the UT26 genome consists three large circular replicons of 3.6 Mb, 670 kb, and 185 kb. The 3.6 Mb and the 670 kb replicons had one and two copies, respectively, of the 16S ribosomal RNA gene, and these replicons were designated as chromosomes (Chr) I and II, respectively. Chr I was indicated to be a main chromosome carrying the dnaA gene. The first three lin genes, linA to linC, for conversion of gamma-HCH to 2, 5-dichlorohydroquinone, were dispersed on Chr I. The 185 kb plasmid, pCHQ1, carried the linRED operon for the conversion of 2,5-dichlorohydroquinone to maleylacetate and was conjugatively transferred to another sphingomonad strain. The linF gene encoding maleylacetate reductase was located on Chr II. These results indicated that the genes for the complete gamma-HCH degradation are dispersed on the three large replicons of UT26. An endogenous insertion sequence of UT26, ISsp1, was identified, which may be involved in the rearrangement of the UT26 genome. Furthermore, it was revealed that metabolites of gamma-HCH had toxic effect on UT26, and a ABC-type transporter which was essential for the utilization of gamma-HCH in UT26 was identified. These results demonstrated the importance of factors except the catalyzing enzymes for the gamma-HCH assimilation in UT26.