The regulatory mechanism of 2,4,6-trichlorophenol catabolic operon expression by HadR in Ralstonia pickettii DTP0602

Ralstonia pickettii DTP0602 utilizes 2,4,6-trichlorophenol (2,4,6-TCP) as its sole source of carbon. The expression of catabolic pathway genes (hadA, hadB and hadC) for 2,4,6-TCP has been reported to be regulated by the LysR-type transcriptional regulator (LTTR) HadR. Generally, coinducers are recognized as being important for the function of LTTRs, and alteration of the LTTR-protection sequence and the degree of DNA bending are characteristic of LTTRs with or without a recognized coinducer. In this study, we describe the mechanism by which HadR regulates the expression of 2,4,6-TCP catabolic genes. The 2,4,6-TCP catabolic pathway genes in DTP0602 consist of two transcriptional units: hadX-hadA-hadB-hadC and monocistronic hadR. Purified HadR binds to the hadX promoter and HadR-DNA complex formation was induced in the presence of 16 types of substituted phenols, including chloro- and nitro-phenols and tribromo-phenol. In contrast with observations of other well-characterized LTTRs, the tested phenols showed no diversity of the bending angle of the HadR binding fragment. The expression of 2,4,6-TCP catabolic pathway genes, which are regulated by HadR, was not influenced by the DNA bending angle of HadR. Moreover, the transcription of hadX, hadA and hadB was induced in the presence of seven types of substituted phenols, whereas the other substituted phenols, which induced formation of the HadR-DNA complex, did not induce the transcription of hadX, hadA or hadB in vivo.