A novel pathway for the biodegradation of gamma-hexachlorocyclohexane by a Xanthomonas sp. strain ICH12.
OBJECTIVE
To isolate gamma-hexachlorocyclohexane (HCH)-degrading bacteria from contaminated soil and characterize the metabolites formed and the genes involved in the degradation pathway.


RESULTS
A bacterial strain Xanthomonas sp. ICH12, capable of biodegrading gamma- HCH was isolated from HCH-contaminated soil. DNA-colony hybridization method was employed to detect bacterial populations containing specific gene sequences of the gamma-HCH degradation pathway. linA (dehydrodehalogenase), linB (hydrolytic dehalogenase) and linC (dehydrogenase) from a Sphingomonas paucimobilis UT26, reportedly possessing gamma-HCH degradation activity, were used as gene probes against isolated colonies. The isolate was found to grow and utilize gamma-HCH as the sole carbon and energy source. The 16S ribosomal RNA gene sequence of the isolate resulted in its identification as a Xanthomonas species, and we designated it as strain ICH12. During the degradation of gamma-HCH by ICH12, formation of two intermediates, gamma-2,3,4,5,6-pentachlorocyclohexene (gamma-PCCH), and 2,5-dichlorobenzoquinone (2,5-DCBQ), were identified by gas chromatography-mass spectrometric (GC-MS) analysis. While gamma-PCCH was reported previously, 2,5-dichlorohydroquinone was a novel metabolite from HCH degradation.


CONCLUSIONS
A Xanthomonas sp. for gamma-HCH degradation from a contaminated soil was isolated. gamma-HCH was utilized as sole source of carbon and energy, and the degradation proceeds by successive dechlorination. Two degradation products gamma-PCCH and 2,5-DCBQ were characterized, and the latter metabolite was not known in contrasts with the previous studies. The present work, for the first time, demonstrates the potential of a Xanthomonas species to degrade a recalcitrant and widespread pollutant like gamma-HCH.


CONCLUSIONS
This study demonstrates the isolation and characterization of a novel HCH-degrading bacterium. Further results provide an insight into the novel degradation pathway which may exist in diverse HCH-degrading bacteria in contaminated soils leading to bioremediation of gamma-HCH.
Manickam
N
N
Environmental Biotechnology, Industrial Toxicology Research Centre, Lucknow, India. nmanickam2005@yahoo.com
Misra
R
R
Mayilraj
S
S
D001483:Base Sequence; D016227:Benzoquinones / Q000378:metabolism; D001673:Biodegradation, Environmental; D003510:Cyclohexanes / Q000378:metabolism; D005798:Genes, Bacterial; D001556:Hexachlorocyclohexane / Q000378:metabolism*; D007306:Insecticides / Q000378:metabolism*; D053858:Metabolic Networks and Pathways; D010088:Oxidoreductases / Q000235:genetics; D016133:Polymerase Chain Reaction / Q000379:methods; D012329:RNA, Bacterial / Q000235:genetics; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D012988:Soil Microbiology; D014974:Xanthomonas / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*
C524862:2,3,4,5,6-pentachlorocyclohexene; C524861:2,5-dichlorobenzoquinone; D016227:Benzoquinones; D003510:Cyclohexanes; D007306:Insecticides; D012329:RNA, Bacterial; D012336:RNA, Ribosomal, 16S; D001556:Hexachlorocyclohexane; D010088:Oxidoreductases