BldR2, a new transcriptional regulator from the archaeon Sulfolobus solfataricus belonging to the MarR family

The multiple antibiotic resistance regulator (MarR) family constitutes a significant class of transcriptional regulators, abundantly distributed throughout the bacterial and archaeal domains. It includes numerous members that control a variety of important biological functions such as environmental surveillance of aromatic compounds, resistance to antimicrobial agents and regulation of aromatic catabolic pathways [1]. In recent years, intensified structural investigations on bacterial and archaeal MarR homologs have contributed to elucidate the mechanistic basis of DNA and ligand binding; but for the majority of the archaeal representatives, the biological function, as well as the definition of the gene targets is still elusive [2]. Here we describe the identification and characterisation of BldR2, a new member of this family from the archaeon Sulfolobus solfataricus. Transcriptional analysis, revealed regulation of BldR2 expression and allowed the identification of cis-acting basal and regulatory sequences. A three-dimensional model of BldR2 showed a structure typical of MarR proteins including the winged-helix DNA binding motif, despite low primary sequence similarity among the members. The recombinant protein is indeed a DNA binding protein which binds to a direct repeat in its own promoter located immediately upstream of the basal regulatory cis-acting sequences. BldR2 also recognises sequences in the putative promoters of the neighbour Sso1080 and Sso1078 genes, coding respectively for a putative permease and the ATP-binding component of the ABC-type antimicrobial peptide transport system. Furthermore, we demonstrate that BldR2 is able to interact with two phenolic ligands, benzaldehyde and salicylate. Characterisation of the interaction of BldR2 proved different binding affinities strongly suggesting that the benzaldehyde is the preferred ligand. From this study we can propose a general regulative mechanism for MarR proteins in the archaea consisting in the surveillance of environmental drugs through specific binding to position-conserved regulatory sequences and consequent activation of transport efflux membrane proteins.