Organization of lin Genes and IS 6100 among Different Strains of Hexachlorocyclohexane-Degrading Sphingomonas paucimobilis : Evidence for Horizontal Gene Transfer

Abstract

The organization of lin genes and IS 6100 was studied in three strains of Sphingomonas paucimobilis (B90A, Sp+, and UT26) which degraded hexachlorocyclohexane (HCH) isomers but which had been isolated at different geographical locations. DNA-DNA hybridization data revealed that most of the lin genes in these strains were associated with IS 6100 , an insertion sequence classified in the IS 6 family and initially found in Mycobacterium fortuitum. Eleven, six, and five copies of IS 6100 were detected in B90A, Sp+, and UT26, respectively. IS 6100 elements in B90A were sequenced from five, one, and one regions of the genomes of B90A, Sp+, and UT26, respectively, and were found to be identical. DNA-DNA hybridization and DNA sequencing of cosmid clones also revealed that S. paucimobilis B90A contains three and two copies of linX and linA , respectively, compared to only one copy of these genes in strains Sp+ and UT26. Although the copy number and the sequence of the remaining genes of the HCH degradative pathway ( linB , linC , linD , and linE ) were nearly the same in all strains, there were striking differences in the organization of the linA genes as a result of replacement of portions of DNA sequences by IS 6100 , which gave them a strange mosaic configuration. Spontaneous deletion of linD and linE from B90A and of linA from Sp+ occurred and was associated either with deletion of a copy of IS 6100 or changes in IS 6100 profiles. The evidence gathered in this study, coupled with the observation that the G+C contents of the linA genes are lower than that of the remaining DNA sequence of S. paucimobilis , strongly suggests that all these strains acquired the linA gene through horizontal gene transfer mediated by IS 6100 . The association of IS 6100 with the rest of the lin genes further suggests that IS 6100 played a role in shaping the current lin gene organization.