Degradation of Beta-Hexachlorocyclohexane by Haloalkane Dehalogenase LinB from Hexachlorocyclohexane-Utilizing Bacterium Sphingobium sp. MI1205
Ito, M., Prokop, Z., Klvana, M., Otsubo, Y., Tsuda, M., Damborsky, J., Nagata, Y.
ARCHIVES OF MICROBIOLOGY 188: 313-325 (2007)
The technical formulation of hexachlorocyclohexane (HCH) mainly consists of the insecticidal gamma-isomer and noninsecticidal alpha, beta, and delta-isomers, among which beta-HCH is the most recalcitrant and has caused serious environmental problems. A gamma-HCH-utilizing bacterial strain, Sphingobium sp. MI1205, was isolated from soil which had been contaminated with HCH isomers. This strain degraded beta-HCH more rapidly than the well-characterized gamma-HCH-utilizing strain Sphingobium japonicum UT26. In MI1205, beta-HCH was converted to 2,3,5,6-tetrachlorocyclohexane-1,4-diol (TCDL) via 2,3,4,5,6-pentachlorocyclohexanol (PCHL). A haloalkane dehalogenase LinB (LinB-MI) that is 98 % identical (seven amino-acid differences among 296 amino acids) to LinB from UT26 (LinB-UT) was identified as an enzyme responsible for the two-step conversion of beta-HCH to TCDL. This property of LinB-MI contrasted with that of LinB-UT, which catalyzed only the first step conversion of beta-HCH to PCHL. Site-directed mutagenesis and computer modeling suggested that two of the seven different amino acid residues (V134 and H247) forming a catalytic pocket of LinB are important for the binding of PCHL in an orientation suitable for the reaction in LinB-MI. However, mutagenesis also indicated the involvement of other residues for the activity unique to LinB-MI. Sequence analysis revealed that MI1205 possesses the IS6100-flanked cluster that contains two copies of the linB-MI gene. This cluster is identical to the one located on the exogenously isolated plasmid pLB1, suggesting that MI1205 had recruited the linB genes by a horizontal transfer event.