Regulation of reductive dehalogenase gene transcription in Dehalococcoides mccartyi

The remarkable capacity of the genus Dehalococcoides to dechlorinate a multitude of different chlorinated organic compounds reflects the number and diversity of genes in the genomes of Dehalococcoides species encoding reductive dehalogenase homologues (rdh). Most of these genes are located in the vicinity of genes encoding multiple antibiotic resistance regulator (MarR)-type or two-component system regulators. Here, the transcriptional response of rdhA genes (coding for the catalytic subunit) to 2,3- and 1,3-dichlorodi-benzo-p-dioxin (DCDD) was studied in Dehalococcoides mccartyi strain CBDB1. Almost all rdhA genes were transcribed in the presence of 2,3-DCDD, albeit at different levels as shown for the transcripts of cbrA, cbdbA1453, cbdbA1624 and cbdbA1588. By contrast, 1,3-DCDD did not induce rdhA transcription. The putative MarR CbdbA1625 was heterologously produced and its ability to bind in vitro to the overlapping promoter regions of the genes cbdbA1624 and cbdbA1625 was demonstrated. To analyse regulation in vivo, single-copy transcriptional promoter-lacZ fusions of different rdhA genes and of cbdbA1625 were constructed and introduced into the heterologous host Escherichia coli, and expression levels of the fusions were measured. The cbdbA1625 gene was cloned into a vector allowing a regulation of expression by arabinose and it was transformed into the strains containing the rdh-promoter-lacZ fusion derivatives. CbdbA1625 was shown to downregulate transcription from its own promoter resulting in a 40-50% reduction in the beta-galactosidase activity, giving the first hint that it acts as a repressor.