Selective loss of lin genes from hexachlorocyclohexane-degrading Pseudomonas aeruginosa ITRC-5 under different growth conditions

Abstract

The chlorinated insecticide gamma-hexachlorocyclohexane (gamma-HCH) is sequentially metabolized by the products of linA, linB, linC, linD, linE, and linF genes to beta-ketoadipate, which is subsequently mineralized. Two or more copies of these genes are present in the bacterium Pseudomonas aeruginosa ITRC-5 that was isolated earlier by selective enrichment on technical-HCH. At least one copy of linA, linB, linC, linD, and possibly linE is lost from ITRC-5 upon its growth on gamma-HCH. All the lin genes, however, are lost when the bacterium was grown in Luria-Bertani (LB) medium. The loss of lin genes is accompanied with the loss/rearrangement of insertion sequence IS6100 genes. Concomitant to the loss of lin genes, the degradation of HCH-isomers by "gamma-HCH grown cells" is slower, when compared with "technical-HCH grown cells", and is completely lost by "LB-grown cells". The selective loss of lin genes during different growth conditions has not been reported before and is expected to help in understanding the dynamism of degradative genes.

Biochemistry of Microbial Degradation of Hexachlorocyclohexane and Prospects for Bioremediation

“…MI1205 and P. aeruginosa ITRC-5 were reported to contain an IS6100-flanked duplication of a linB variant similar to the linB sequences found in strains B90A and BHC-A (62,161). This cluster is also identical to the one found for plasmid pLB1, isolated from soil, suggesting a horizontal transfer of lin genes (103).…”

“…Like linA and linB, linC was found to be constitutively expressed in strains UT26 and B90A (115,168). More divergent (97 to 98% amino acid identities) linC genes have also been recovered from HCH-degrading Pseudomonas aeruginosa ITRC-5 (161) and Microbacterium sp. ITRC1 (88).…”

“…Most strains tested have one copy of the linA gene, but B90A and Pseudomonas aeruginosa ITRC-5 are known to have two, denoted linA1/linA2 and linAa/linAb (40,72,75,161). There is also evidence from DNA hybridization studies that some other strains may also have two copies of the linA gene (31).…”

“…Similarly, although the organizations of the genes are quite variable among species (see below), this variation bears no relationship to the phylogeny of the organisms. For example, linA, linB, and linDER from Pseudomonas aeruginosa ITRC-5 show microsyntenous arrangements that are strikingly similar to those of their counterparts in UT26 and B90A (161).…”

The lin Genes for γ-Hexachlorocyclohexane Degradation in Sphingomonas sp. MM-1 Proved to Be Dispersed across Multiple Plasmids

“…Considering the difficulty of isolating MM-1 from the mixed culture, MTB-1 might have some functions supporting the growth of MM-1 in -HCH minimal medium. However, another possibility cannot be excluded that MTB-1 also maintains the lin genes only in -HCH minimal medium, as suggested by Kumar et al 31) Elucidation of a mechanism for the coexistence of the two strains in -HCH minimal medium would be of great interest.…”