A gene linB2 responsible for the conversion of β-HCH and 2,3,4,5,6-pentachlorocyclohexanol in Sphingomonas sp. BHC-A

Adaptive evolution of Sphingopyxis sp. MC4 conferred degradation potential for persistent β- and δ-Hexachlorocyclohexane (HCH) isomers

Hexachlorocyclohexane (HCH), an organochlorine pesticide imposes several harmful impacts on the ecosystem. β- and δ-isomers of HCH are highly toxic, persistent, and recalcitrant to biodegradation, slow and incomplete degradation of β- and δ- isomers have been reported in a few strains. We have isolated a strain designated as Sphingopyxis strain MC4 that can tolerate and degrade high concentrations of α-, β-, γ- and δ-HCH isomers. To date, no other Sphingopyxis strain has been reported to degrade β- and δ-isomers. To understand the underlying genetic makeup contributing to adaptations, the whole genome of strain MC4 was sequenced. Comparative genome analysis showed that strain MC4 harbors the complete pathway (lin genes) required for HCH degradation. Genetic footprints such as presence of lin genes on genomic islands, IS6100 elements in close proximity of lin genes, and synteny in lin flanking regions with other strains reflects the horizontal gene transfer in strain MC4. Positive selection and HGT drive the adaptive evolution of strain MC4 under the pressure of HCH contamination that it experienced in its surrounding niche. In silico analyses showed efficient binding of β- and δ-isomers with enzymes leading to rapid degradation that need further validation by cloning and biochemical experiments.

Soil from a Hexachlorocyclohexane Contaminated Field Site Inoculates Wheat in a Pot Experiment to Facilitate the Microbial Transformation of β-Hexachlorocyclohexane Examined by Compound-Specific Isotope Analysis

β-Hexachlorocyclohexane (β-HCH) is a remnant from former HCH pesticide production. Its removal from the environment gained attention in the last few years since it is the most stable HCH isomer. However, knowledge about the transformation of β-HCH in soil-plant systems is still limited. Therefore, experiments with a contaminated field soil were conducted to investigate the transformation of β-HCH in soil-plant systems by compound specific isotope analysis (CSIA). The results showed that the δ¹³C and δ³⁷Cl values of β-HCH in the soil of the planted control remained stable, revealing no transformation due to a low bioavailability. Remarkably, an increase of the δ¹³C and δ³⁷Cl values in soil and plant tissues of the spiked treatments were observed, indicating the transformation of β-HCH in both the soil and the plant. This was surprising as previously it was shown that wheat is unable to transform β-HCH when growing in hydroponic culture or garden soil. Thus, results of this work indicate for the first time that a microbial community of the soil inoculated the wheat and then facilitated the transformation of β-HCH in the wheat, which may have implications for the development of phytoremediation concepts. A high abundance of HCH degraders belonging to Sphingomonas sp., Mycobacterium sp., and others was detected in the β-HCH-treated bulk and rhizosphere soil, potentially supporting the biotransformation.

Microbial catabolism of lindane in distinct layers of acidic paddy soils combinedly affected by different water managements and bioremediation strategies

The environmental fate of the recalcitrant organic chlorine insecticide lindane and its removal from contaminated soils are still of great concern. However, the key factors influencing microbial removal of lindane from paddy soils with intermittent flooding and draining remain largely unknown. Here, we conducted laboratory experiments to investigated lindane biodegradation in different layers of typical acidic paddy soils under different water managements and bioremediation strategies, together with the changes of functional bacterial consortium, key genes and metabolic pathways. It was found that under flooded conditions, lindane spiking significantly stimulated the growth of some bacterial genera with potential anaerobic catabolic functions in both top- (0-20 cm depth) and subsoil (20-40 cm depth), leading to the shortest half-life of lindane with 7.6-9.0 d in the topsoil. In contrary, lindane spiking dramatically stimulated the growth of bacterial members with aerobic catabolic functions under drained conditions, exhibiting half-lives of lindane with 85-131 d and 18-23 d in the top- and subsoil, respectively. Overall, biostimulation coupled with flooding management would be the better combination for increased lindane bioremediation. Functional genes involved in lindane degradation and retrieved from metagenomic data further supported the anaerobic and aerobic biodegradation of lindane under flooded and drained conditions, respectively. Moreover, the integrated network analysis suggested water management and organic matter were the primary factors shaped the assembly of functional bacteria in lindane degradation, among which Clostridium and Rhodanobacter were the key anaerobic and aerobic functional genera, respectively. Taken together, our study provides a comprehensive understanding of lindane biodegradation in distinct layers of acidic paddy soils that were combinedly affected by different water managements and bioremediation strategies.

Lessons from the genomes of lindane-degrading sphingomonads

Bacterial strains capable of degrading man-made xenobiotic compounds are good materials to study bacterial evolution towards new metabolic functions. Lindane (γ-hexachlorocyclohexane, γ-HCH, or γ-BHC) is an especially good target compound for the purpose, because it is relatively recalcitrant but can be degraded by a limited range of bacterial strains. A comparison of the complete genome sequences of lindane-degrading sphingomonad strains clearly demonstrated that (i) lindane-degrading strains emerged from a number of different ancestral hosts that have recruited lin genes encoding enzymes that are able to channel lindane to central metabolites, (ii) in sphingomonads lin genes have been acquired by horizontal gene transfer mediated by different plasmids and in which IS6100 plays a role in recruitment and distribution of genes, and (iii) IS6100 plays a role in dynamic genome rearrangements providing genetic diversity to different strains and ability to evolve to other states. Lindane-degrading bacteria whose genomes change so easily and quickly are also fascinating starting materials for tracing the bacterial evolution process experimentally in a relatively short time period. As the origin of the specific lin genes remains a mystery, such genes will be useful probes for exploring the cryptic 'gene pool' available to bacteria.

Comparative genomic analysis of 26 Sphingomonas and Sphingobium strains: Dissemination of bioremediation capabilities, biodegradation potential and horizontal gene transfer

Bacteria belonging to the genera Sphingomonas and Sphingobium are known for their ability to catabolize aromatic compounds. In this study, we analyzed the whole genome sequences of 26 strains in the genera Sphingomonas and Sphingobium to gain insight into dissemination of bioremediation capabilities, biodegradation potential, central pathways and genome plasticity. Phylogenetic analysis revealed that both Sphingomonas sp. strain BHC-A and Sphingomonas paucimobilis EPA505 should be placed in the genus Sphingobium. The bph and xyl gene cluster was found in 6 polycyclic aromatic hydrocarbons-degrading strains. Transposase and IS coding genes were found in the 6 gene clusters, suggesting the mobility of bph and xyl gene clusters. β-ketoadipate and homogentisate pathways were the main central pathways in Sphingomonas and Sphingobium strains. A large number of oxygenase coding genes were predicted in the 26 genomes, indicating a huge biodegradation potential of the Sphingomonas and Sphingobium strains. Horizontal gene transfer related genes and prophages were predicted in the analyzed strains, suggesting the ongoing evolution and shaping of the genomes. Analysis of the 26 genomes in this work contributes to the understanding of dispersion of bioremediation capabilities, bioremediation potential and genome plasticity in strains belonging to the genera Sphingomonas and Sphingobium.

Evidence of α-, β- and γ-HCH mixture aerobic degradation by the native actinobacteria Streptomyces sp. M7

The organochlorine insecticide γ-hexachlorocyclohexane (γ-HCH, lindane) and its non-insecticidal α- and β-isomers continue to pose serious environmental and health concerns, although their use has been restricted or completely banned for decades. In this study we report the first evidence of the growth ability of a Streptomyces strain in a mineral salt medium containing high doses of α- and β-HCH (16.6 mg l(-1)) as a carbon source. Degradation of HCH isomers by Streptomyces sp. M7 was investigated after 1, 4, and 7 days of incubation, determining chloride ion release, and residues in the supernatants by GC with µECD detection. The results show that both the α- and β-HCH isomers were effectively metabolized by Streptomyces sp. M7, with 80 and 78 % degradation respectively, after 7 days of incubation. Moreover, pentachlorocyclohexenes and tetrachlorocyclohexenes were detected as metabolites. In addition, the formation of possible persistent compounds such as chlorobenzenes and chlorophenols were studied by GC-MS, while no phenolic compounds were detected. In conclusion, we have demonstrated for the first time that Streptomyces sp. M7 can degrade α- and β-isomers individually or combined with γ-HCH and could be considered as a potential agent for bioremediation of environments contaminated by organochlorine isomers.

Comparative genomic analysis of nine Sphingobium strains: insights into their evolution and hexachlorocyclohexane (HCH) degradation pathways

BACKGROUND

Sphingobium spp. are efficient degraders of a wide range of chlorinated and aromatic hydrocarbons. In particular, strains which harbour the lin pathway genes mediating the degradation of hexachlorocyclohexane (HCH) isomers are of interest due to the widespread persistence of this contaminant. Here, we examined the evolution and diversification of the lin pathway under the selective pressure of HCH, by comparing the draft genomes of six newly-sequenced Sphingobium spp. (strains LL03, DS20, IP26, HDIPO4, P25 and RL3) isolated from HCH dumpsites, with three existing genomes (S. indicum B90A, S. japonicum UT26S and Sphingobium sp. SYK6).

RESULTS

Efficient HCH degraders phylogenetically clustered in a closely related group comprising of UT26S, B90A, HDIPO4 and IP26, where HDIPO4 and IP26 were classified as subspecies with ANI value >98%. Less than 10% of the total gene content was shared among all nine strains, but among the eight HCH-associated strains, that is all except SYK6, the shared gene content jumped to nearly 25%. Genes associated with nitrogen stress response and two-component systems were found to be enriched. The strains also housed many xenobiotic degradation pathways other than HCH, despite the absence of these xenobiotics from isolation sources. Additionally, these strains, although non-motile, but posses flagellar assembly genes. While strains HDIPO4 and IP26 contained the complete set of lin genes, DS20 was entirely devoid of lin genes (except linKLMN) whereas, LL03, P25 and RL3 were identified as lin deficient strains, as they housed incomplete lin pathways. Further, in HDIPO4, linA was found as a hybrid of two natural variants i.e., linA1 and linA2 known for their different enantioselectivity.

CONCLUSION

The bacteria isolated from HCH dumpsites provide a natural testing ground to study variations in the lin system and their effects on degradation efficacy. Further, the diversity in the lin gene sequences and copy number, their arrangement with respect to IS6100 and evidence for potential plasmid content elucidate possible evolutionary acquisition mechanisms for this pathway. This study further opens the horizon for selection of bacterial strains for inclusion in an HCH bioremediation consortium and suggests that HDIPO4, IP26 and B90A would be appropriate candidates for inclusion.

Draft Genome Sequence of Sphingobium sp. Strain BHC-A, Revealing Genes for the Degradation of Hexachlorocyclohexane

Here, we report the draft genome sequence of Sphingobium sp. strain BHC-A, a lin gene-based hexachlorocyclohexane (HCH)-degrading strain, isolated from soil that suffered long-term HCH contamination in an insecticide factory.

Crystal structure and site-directed mutagenesis analyses of haloalkane dehalogenase LinB from Sphingobium sp. strain MI1205

The enzymes LinB(UT) and LinB(MI) (LinB from Sphingobium japonicum UT26 and Sphingobium sp. MI1205, respectively) catalyze the hydrolytic dechlorination of β-hexachlorocyclohexane (β-HCH) and yield different products, 2,3,4,5,6-pentachlorocyclohexanol (PCHL) and 2,3,5,6-tetrachlorocyclohexane-1,4-diol (TCDL), respectively, despite their 98% identity in amino acid sequence. To reveal the structural basis of their different enzymatic properties, we performed site-directed mutagenesis and X-ray crystallographic studies of LinB(MI) and its seven point mutants. The mutation analysis revealed that the seven amino acid residues uniquely found in LinB(MI) were categorized into three groups based on the efficiency of the first-step (from β-HCH to PCHL) and second-step (from PCHL to TCDL) conversions. Crystal structure analyses of wild-type LinB(MI) and its seven point mutants indicated how each mutated residue contributed to the first- and second-step conversions by LinB(MI). The dynamics simulation analyses of wild-type LinB(MI) and LinB(UT) revealed that the entrance of the substrate access tunnel of LinB(UT) was more flexible than that of LinB(MI), which could lead to the different efficiencies of dehalogenation activity between these dehalogenases.

Biodegradation of α-, β-, and γ-hexachlorocyclohexane by Arthrobacter fluorescens and Arthrobacter giacomelloi

The organochlorine pesticide γ-hexachlorocyclohexane (γ-HCH, lindane) and its non-insecticidal isomers α-, β-, and δ- continue to pose serious environmental and health concerns, although their use has been restricted or completely banned for decades. The present study reports the first results on the ability of two Arthrobacter strains, not directly isolated from a HCH-polluted site, to grow in a mineral salt medium containing α-, β-, or γ-HCH (100 mgl(-1)) as sole source of carbon. Growth of cultures and HCHs degradation by Arthrobacter fluorescens and Arthrobacter giacomelloi were investigated after 1, 2, 3, 4, and 7 days of incubation by enumerating colony forming units and GC with ECD detection, respectively. Both bacteria are able to metabolize the HCHs: A. giacomelloi is the most effective one, as after 72 h of incubation it produces 88 % degradation of α-, 60 % of β-, and 56 % of γ-HCH. The formation of possible persistent compounds was studied by GC/MS and by HPLC analysis. Pentachlorocyclohexenes and tetrachlorocyclohexenes have been detected as metabolites, which are almost completely eliminated after 72 h of incubation, while no phenolic compounds were found.

Bacterial bio-resources for remediation of hexachlorocyclohexane

In the last few decades, highly toxic organic compounds like the organochlorine pesticide (OP) hexachlorocyclohexane (HCH) have been released into the environment. All HCH isomers are acutely toxic to mammals. Although nowadays its use is restricted or completely banned in most countries, it continues posing serious environmental and health concerns. Since HCH toxicity is well known, it is imperative to develop methods to remove it from the environment. Bioremediation technologies, which use microorganisms and/or plants to degrade toxic contaminants, have become the focus of interest. Microorganisms play a significant role in the transformation and degradation of xenobiotic compounds. Many Gram-negative bacteria have been reported to have metabolic abilities to attack HCH. For instance, several Sphingomonas strains have been reported to degrade the pesticide. On the other hand, among Gram-positive microorganisms, actinobacteria have a great potential for biodegradation of organic and inorganic toxic compounds. This review compiles and updates the information available on bacterial removal of HCH, particularly by Streptomyces strains, a prolific genus of actinobacteria. A brief account on the persistence and deleterious effects of these pollutant chemical is also given.

Biochemistry of microbial degradation of hexachlorocyclohexane and prospects for bioremediation

Lindane, the gamma-isomer of hexachlorocyclohexane (HCH), is a potent insecticide. Purified lindane or unpurified mixtures of this and alpha-, beta-, and delta-isomers of HCH were widely used as commercial insecticides in the last half of the 20th century. Large dumps of unused HCH isomers now constitute a major hazard because of their long residence times in soil and high nontarget toxicities. The major pathway for the aerobic degradation of HCH isomers in soil is the Lin pathway, and variants of this pathway will degrade all four of the HCH isomers although only slowly. Sequence differences in the primary LinA and LinB enzymes in the pathway play a key role in determining their ability to degrade the different isomers. LinA is a dehydrochlorinase, but little is known of its biochemistry. LinB is a hydrolytic dechlorinase that has been heterologously expressed and crystallized, and there is some understanding of the sequence-structure-function relationships underlying its substrate specificity and kinetics, although there are also some significant anomalies. The kinetics of some LinB variants are reported to be slow even for their preferred isomers. It is important to develop a better understanding of the biochemistries of the LinA and LinB variants and to use that knowledge to build better variants, because field trials of some bioremediation strategies based on the Lin pathway have yielded promising results but would not yet achieve economic levels of remediation.

Proposal of biostimulation for hexachlorocyclohexane (HCH)-decontamination and characterization of culturable bacterial community from high-dose point HCH-contaminated soils

AIMS

To locate a high-dose point hexachlorocyclohexane (HCH)-contaminated site, to identify HCH-degrading bacteria in it and assay HCH-decontamination by biostimulation.

METHODS AND RESULTS

Bacteria were isolated by serial dilution method from HCH-contaminated soil samples collected from areas near an HCH-manufacturing unit and its dumpsite in North India. After confirming the presence of indigenous HCH-degraders (seven of 24 strains), an ex situ biostimulation experiment was conducted. For this, residue levels in soil were diluted by mixing with pristine garden soil and aeration, moisture and nutrients were provided intermittently. This soil was monitored for reduction in Sigma-HCH (sum of alpha-, beta-, gamma- and delta-HCH) levels and stimulation of HCH-degraders. Experiments were conducted twice, in March-April (c. 75 microg Sigma-HCH g(-1) soil) and October-November 2006 (c. 280 microg Sigma-HCH g(-1) soil) at 26-30 degrees C. Sigma-HCH levels were reduced to <30% of the original in 24 days and <3% in 240 days in the experimental pits. Terminal restriction fragment length polymorphism analysis reflected changes in microbial community structure during the course of experiment.

CONCLUSIONS

Our results show presence of HCH-degrading sphingomonads at a high-dose point HCH-contaminated site and presents biostimulation as an effective approach for its decontamination via aeration, addition of nutrients and moisture, of the indigenous population.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study demonstrates that biostimulation of indigenous HCH-degrading microbial population can be used for decontamination of chronically HCH-contaminated sites.

New metabolites in the degradation of alpha- and gamma-hexachlorocyclohexane (HCH): pentachlorocyclohexenes are hydroxylated to cyclohexenols and cyclohexenediols by the haloalkane dehalogenase LinB from Sphingobium indicum B90A

Technical hexachlorocyclohexane (HCH) and lindane are obsolete pesticides whose former production and use led to widespread contaminations posing serious and lasting health and environmental risks. Out of nine possible stereoisomers, alpha-, beta-, gamma-, and delta-HCH are usually present at contaminated sites, and research for a better understanding of their biodegradation has become essential for the development of appropriate remediation technologies. Because haloalkane dehalogenase LinB was recently found responsible for the hydroxylation of beta-HCH, delta-HCH, and delta-pentachlorocyclohexene (delta-PCCH), we decided to examine whether beta- and gamma-PCCH, which can be formed by LinA from alpha- and gamma-HCH, respectively, were also converted by LinB. Incubation of such substrates with Escherichia coli BL21 expressing functional LinB originating from Sphingobium indicum B90A showed that both beta-PCCH and gamma-PCCH were direct substrates of LinB. Furthermore, we identified the main metabolites as 3,4,5,6-tetrachloro-2-cyclohexene-1-ols and 2,5,6-trichloro-2-cyclohexene-1,4-diols by nuclear magnetic resonance spectroscopy and gas chromatography-mass spectrometry. In contrast to alpha-HCH, gamma-HCH was not a substrate for LinB. On the basis of our data, we propose a modified gamma-HCH degradation pathway in which gamma-PCCH is converted to 2,5-cyclohexadiene-1,4-diol via 3,4,5,6-tetrachloro-2-cyclohexene-1-ol and 2,5,6-trichloro-2-cyclohexene-1,4-diol.

Pseudomonas sp. to Sphingobium indicum: a journey of microbial degradation and bioremediation of Hexachlorocyclohexane

The unusual process of production of hexachlorocyclohexane (HCH) and extensive use of technical HCH and lindane has created a very serious problem of HCH contamination. While the use of technical HCH and lindane has been banned all over the world, India still continues producing lindane. Bacteria, especially Sphingomonads have been isolated that can degrade HCH isomers. Among all the bacterial strains isolated so far, Sphingobium indicum B90A that was isolated from HCH treated rhizosphere soil appears to have a better potential for HCH degradation. This conclusion is based on studies on the organization of lin genes and degradation ability of B90A. This strain perhaps can be used for HCH decontamination through bioaugmentation.

Analysis of the role of LinA and LinB in biodegradation of ?-hexachlorocyclohexane

Commercial formulations of hexachlorocyclohexane (HCH) consist of a mixture of four isomers, alpha, beta, gamma and delta. All these four isomers are toxic and recalcitrant pollutants. Sphingobium (formerly Sphingomonas) sp. strain BHC-A is able to degrade all four HCH isomers. Eight lin genes responsible for the degradation of gamma-HCH in BHC-A were cloned and analysed for their role in the degradation of delta-HCH, and the initial conversion steps in delta-HCH catabolism by LinA and LinB in BHC-A were found. LinA dehydrochlorinated delta-HCH to produce 1,3,4,6-tetrachloro-1,4-cyclohexadiene (1,4-TCDN) via delta-pentachlorocyclohexene (delta-PCCH). Subsequently, both 1,4-TCDN and delta-PCCH are catalysed by LinB via two successive rounds of hydrolytic dechlorinations to form 2,5-dichloro-2,5-cyclohexadiene-1,4-diol (2,5-DDOL) and 2,3,5-trichloro-5-cyclohexene-1,4-diol (2,3,5-TCDL) respectively. LinB could also catalyse the hydrolytic dechlorination of delta-HCH to 2,3,5,6-tetrachloro-1,4-cyclohexanediol (TDOL) via 2,3,4,5,6-pentachlorocyclohexanol (PCHL).

Aerobic degradation of lindane (gamma-hexachlorocyclohexane) in bacteria and its biochemical and molecular basis

gamma-Hexachlorocyclohexane (gamma-HCH, also called gamma-BHC and lindane) is a halogenated organic insecticide that causes serious environmental problems. The aerobic degradation pathway of gamma-HCH was extensively revealed in bacterial strain Sphingobium japonicum (formerly Sphingomonas paucimobilis) UT26. gamma-HCH is transformed to 2,5-dichlorohydroquinone through sequential reactions catalyzed by LinA, LinB, and LinC, and then 2,5-dichlorohydroquinone is further metabolized by LinD, LinE, LinF, LinGH, and LinJ to succinyl-CoA and acetyl-CoA, which are metabolized in the citrate/tricarboxylic acid cycle. In addition to these catalytic enzymes, a putative ABC-type transporter system encoded by linKLMN is also essential for the gamma-HCH utilization in UT26. Preliminary examination of the complete genome sequence of UT26 clearly demonstrated that lin genes for the gamma-HCH utilization are dispersed on three large circular replicons with sizes of 3.5 Mb, 682 kb, and 191 kb. Nearly identical lin genes were also found in other HCH-degrading bacterial strains, and it has been suggested that the distribution of lin genes is mainly mediated by insertion sequence IS6100 and plasmids. Recently, it was revealed that two dehalogenases, LinA and LinB, have variants with small number of amino acid differences, and they showed dramatic functional differences for the degradation of HCH isomers, indicating these enzymes are still evolving at high speed.

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

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.

Degradation of beta-hexachlorocyclohexane by haloalkane dehalogenase LinB from gamma-hexachlorocyclohexane-utilizing bacterium Sphingobium sp. MI1205

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.