ChpR Is a Chlorpyrifos-Responsive Transcription Regulator in <i>Sinorhizobium</i> <i>meliloti</i>

The broad-spectrum organophosphate insecticide chlorpyrifos (CPF)-inducible locus, <i>chpAB,</i> was identified on the endogenous plasmid pSymB in <i>Sinorhizobium meliloti</i>. The <i>S. meliloti chpA</i> promoter was highly induced by CPF and was induced at much lower levels by diazinon and ethion. Transcription of <i>chpA</i> was dependent on <i>chpR,</i> a CadC family transcriptional regulator located upstream of, and divergently transcribed from, <i>chpAB.</i> ChpR was able to mediate the CPF-inducible expression of the <i>S. meliloti</i><i>chpA</i> promoter in <i>Escherichia coli</i> through direct interaction with the <i>chpAB</i> promoter. The <i>chpR-chpA</i> intergenic regions of several bacterial <i>chpRAB</i> operons were aligned and a putative ChpR-binding sequence was proposed. Both the ChpR transcription factor and <i>chpA </i>promoter constitute a good candidate system for genetic-based biosensor development.