The complete genome sequencing of a gamma-hexachlorocyclohexane-degrading strain, Sphingobium japonicum UT26, revealed that the genome consists of two circular chromosomes [with sizes of 3.5 Mb (Chr1) and 682 kb (Chr2)], a 191-kb large plasmid (pCHQ1), and two small plasmids with sizes of 32 and 5 kb. The lin genes are dispersed on Chr1, Chr2, and pCHQ1. Comparison of the UT26 genome with those of other sphingomonad strains demonstrated that the "specific" lin genes for conversion of gamma-HCH to beta-ketoadipate (linA, linB, linC, linRED, and linF) are located on the DNA regions unique to the UT26 genome, suggesting the acquisition of these fin genes by horizontal transfer events. On the other hand, linGHIJ and linKLMN are located on the regions conserved in the genomes of sphingomonads, suggesting that the linGHIJ-encoded beta-ketoadipate pathway and the LinKLMN-type ABC transporter system are involved in core functions of sphingomonads. Based on these results, we propose a hypothesis that UT26 was created by recruiting the specific fin genes into a strain having core functions of sphingomonads. Most of the specific lin genes in UT26 are associated with IS6100. Our analysis of spontaneous linA-, linC-, and linRED- deletion mutants of UT26 revealed the involvement of IS6100 in their deduced genome rearrangements. These facts strongly suggest that IS6100 plays important roles both in the dissemination of the specific fin genes and in the genome rearrangements. (C) 2011 Elsevier Inc. All rights reserved.