Elucidation of the functional metal binding profile of a Cd II/PbII sensor CmtRSc from streptomyces coelicolor

Metal homeostasis and resistance in bacteria is maintained by a panel of metal-sensing transcriptional regulators that collectively control transition metal availability and mediate resistance to heavy metal xenobiotics, including AsIII, CdII, PbII, and HgII. The ArsR family constitutes a superfamily of metal sensors that appear to conform to the same winged helical, homodimeric fold, that collectively "sense" a wide array of beneficial metal ions and heavy metal pollutants. The genomes of many actinomycetes, including the soil dwelling bacterium Streptomyces coelicolor and the human pathogen Mycobacterium tuberculosis, encode over ten ArsR family regulators, most of unknown function. Here, we present the characterization of a homologue of M. tuberculosis CmtR (CmtRMtb) from S. coelicolor, denoted CmtRSc. We show that CmtRSc, in contrast to CmtRMtb, binds two monomer mol equivalents of Pb II or CdII to form two pairs of sulfur-rich coordination complexes per dimer. Metal site 1 conforms exactly to the α4C site previously characterized in CmtRMtb while metal site 2 is coordinated by a C-terminal vicinal thiolate pair, Cys110 and Cys111. Biological assays reveal that only CdII and, to a lesser extent, PbII mediate transcriptional derepression in the heterologous host Mycobacterium smegmatis in a way that requires metal site 1. In contrast, mutagenesis of metal site 2 ligands Cys110 or Cys111 significantly reduces CdII responsiveness, with no detectable effect on PbII sensing. The implications of these findings on the ability to predict metal specificity and function from metal-site signatures in the primary structure of ArsR family proteins are discussed. © 2010 American Chemical Society.