Trichloroethene reductive dehalogenase from Dehalococcoides ethenogenes: Sequence of tceA and substrate range characterization

The anaerobic bacterium Dehalococcoides ethenogenes is the only known organ ism that can completely dechlorinate tetrachloroethene or trichloroethene ( TCE) to ethene via dehalorespiration. One of two corrinoid-containing enzym es responsible for this pathway, TCE reductive dehalogenase (TCE-RDase) cat alyzes the dechlorination of TCE to ethene. TCE-RDase dehalogenated 1,2-dic hloroethane and 1,2-dibromoethane to ethene at rates of 7.5 and 30 mu mol/m in/mg, respectively, similar to the rates for TCE, cis-dichloroethene (DCE) , and 1,1-DCE. A variety of other haloalkanes and haloalkenes containing th ree to five carbon atoms were dehalogenated at lower rates. The gene encodi ng TCE-RDase, tceA, was cloned and sequenced via an inverse PCR approach. S equence comparisons of tceA to proteins in the public databases revealed we ak sequence similarity confined to the C-terminal region, which contains th e eight-iron ferredoxin cluster binding motif, (CXXCXXCXXXCP)(2). Direct N- terminal sequencing of the mature enzyme indicated that the first 42 amino acids constitute a signal sequence containing the twin-arginine motif, RRXF XK, associated with the Sec-independent membrane translocation system. This information coupled with membrane localization studies indicated that TCE- RDase is located on the exterior of the cytoplasmic membrane. Like the case for the two other RDases that have been cloned and sequenced, a small open reading frame, tceB, is proposed to be involved with membrane association of TCE-RDase and is predicted to be cotranscribed with tceA.