TtSmtB, an arsenic sensing transcriptional repressor, mediates cadmium resistance in Thermus thermophilus HB27.

The ArsR/SmtB family of transcriptional regulators consists of metal responsive DNA binding proteins sharing a dimeric structure, a helix-turn-helix (HTH) or winged HTH DNA binding domain, and a metal binding box ELCV(C/G)D located within the HTH region. In microbial cells binding of these proteins to different heavy metals determines protein dissociation from target promoters and transcriptional activation. Metal binding mode and specificity vary very much among family members [1]. In a previous study we characterized TtSmtB from the thermophilic microorganism Thermus thermophilus HB27 through in vivo and in vitro approaches, showing that the protein is an arsenic responsive transcriptional repressor which regulates the downstream gene TTC0354 encoding a Zn2+/Cd2+ dependent membrane ATPase involved in arsenic transport outside the cell [2]. In the present study, we further characterized TtSmtB by an array of biochemical and biophysical approaches to deeply analyse its interaction with metal ions (different from arsenic) and target DNA; we show that the protein possesses significant thermal stability and binds other metal ions, such as cadmium and antimony with different affinity. To study the contribution of TtSmtB and TTC0354 in cadmium resistance in vivo, we generated transcriptional fusions of TtSmtB regulatory regions to LacZ in T. thermophilus HB27 wild type and a mutant strain in which TTC0354 was inactivated. These findings revealed the contribution of the system in cadmium resistance opening up to the realization of robust whole-cell biosensors for cadmium detection.