The hdhA gene encodes a haloacid dehalogenase that is regulated by the LysR-type regulator, HdhR, in Sinorhizobium meliloti.

The plasmid pSymA, in the nitrogen-fixing soil bacterium, Sinorhizobium meliloti, carries a 750-bp ORF (SMa1978) designated, hdhA, which encodes a novel dehalogenase that can detoxify haloacid compounds, showing a preference for haloacetic acids. Purified His-tagged HdhA demonstrated the apparent ability to dehalogenate chloroacetic acid and trifluoroacetic acid. In addition, upstream of hdhA, a gene encoding a lysR-type transcription regulator denoted, hdhR (SMa1979), has been identified to be a transcriptional repressor of hdhA expression. In an hdhR knockout mutant, hdhA promoter activity was markedly increased. Purified 32-kDa His-tagged HdhR repressed expression of hdhA by specifically binding to the promoter region of hdhA, as demonstrated by gel mobility shift assay and DNase I foot printing experiments. Moreover, the pesticide, pentachlorophenol, was also found to induce hdhA expression via HdhR. Site-directed mutants, in which the Cys residues at positions 160 and 192 in HdhR were changed to Ser, were constructed. C160S and C192S single mutants showed diminished HdhR-mediated repression of hdhA expression, while a C160S:C192S double mutant could no longer repress expression of hdhA.