Orizontal transfer of haloalkane dehalogenase (LinB) gene among novel hexachlorocyclohexane (HCH) degrading sphingomonads isolated from contaminated soil

Abstract

Soil samples from a HCH dump site and agricultural fields were collected from northern part of India in order to determine residue levels of a-, b-, g- and d- HCH and distribution of HCH degrading sphingomonads. The concentration of HCH residues was found to be very high in dump site soil (upto 4252.47µg/g) but it was low in the soil collected from the agricultural lands (upto 7.7µg/g). Eight novel HCH degrading strains could be isolated from HCH dump site soil but none from agricultural soil. Southern blot hybridization studies revealed the presence of haloalkane dehalogenase (LinB) genes alongwith IS6100 mobile genetic element in all of these strains. Further, 16S rDNA based phylogenetic analysis showed that these strains belong to the genus Sphingobium of family sphingomonadacae. An evidence for horizontal transfer of linB gene among these novel sphingomonads (only five were analysed) was derived by phylogenetic incongruency based approach. The sequence analysis of linB gene of novel isolates showed high sequence similarity (>98.9 to 100%) with each other indicating they were the homologs of same gene. Comparison of “species tree” with “linB gene tree” revealed incongruency among both the trees thus proving that this gene has disseminated among these novel sphingomonads in a recent HGT event. Presence of IS6100 element in all the strains further supported the evidence. This study reveals that the members of family sphingomonadacae are acquiring lin genes by horizontal transfer under high selective pressure of HCH and such phenomenon at a contaminated site is quite significant for bioremediation of HCH.