In Pseudomonas paucimobilis UT26, y-hexachlorocyclohexane (y-HCH) is converted to 2,5-dichloro-2,5-cyclohexadiene-1,4-diol (2,5-DDOL), which is then metabolized to 2,5-dichlorohydroquinone. Here, we isolated from the genomic library of UT26 two genes which expressed 2,5-DDOL dehydrogenase activity when they were transformed into P. putida and Escherichia coli. Both (Fig. 1): y-HCH is converted by two steps of dehydrochlorination via -y-pentachlorocyclohexene (,y-PCCH) to 1,3,4,6-tetrachloro-1,4-cyclohexadiene (1,4-TCDN), which is then metabolized to 2,5-dichloro-2,5-cyclohexadiene-1,4-diol (2,5-DDOL) by two steps of hydrolytic dehalogenation via 2,4,5-trichloro-2,5-cyclohexadiene-1-ol (2,4,. Through this series of reactions, two dead end products, 1,2,4-trichlorobenzene (1,2,4-TCB) and 2,5-dichlorophenol (2,5-DCP), are also produced from unstable intermediates, 1,4-TCDN' and 2,4,5-DNOL, respectively. 2,5-DDOL is further degraded to 2,5-DCHQ (22) and then to a metabolite which is undetectable by gas chromatography (GC) with an electron capture detector, and, finally, 2,5-DCHQ is mineralized (22).In previous studies, we cloned and sequenced two genes involved in the early steps of -y-HCH degradation in UT26 (11,25). The linA gene encodes -y-HCH dehydrochlorinase (LinA), which converts y-HCH to 1,2,4-TCB via -y-PCCH (and 1,4-TCDN) (11). The linA gene was highly expressed in Escherichia coli and was purified to homogeneity (23). LinA released three chloride ions from one molecule of -y-HCH (23). Degradation assays of halogenated compounds by purified LinA showed that the substrate specificity of LinA is very narrow