Identification of a Novel Transcription Regulator from Proteus mirabilis, PMTR, Revealed a Possible Role of YJAI Protein in Balancing Zinc in Escherichia coli

Zinc is an essential trace element required for structural integrity and functional activity of numerous proteins, yet mechanisms by which cells regulate zinc concentration are poorly understood. Here, we identified a gene from Proteus mirabilis that encodes a 135-amino acid residue protein, PMTR (P. mirabilis transcription regulator), a new member of the MerR family of transcription activators. Transformation of Escherichia coli with PMTR-carrying vectors specifically increases cell tolerance to zinc, suggesting the role of PMTR in zinc homeostasis. In response to zinc, PMTR-containing cells robustly accumulate a 12-kDa protein, the amount of which correlates with the cells' ability to grow at high zinc concentrations. The 12-kDa protein is not induced in the presence of Ni2+, Co2+, Cd2+, Mn2+, or Fe2+, indicating that the PMTR-dependent expression of the 12-kDa protein is specifically regulated by zinc. The 12-kDa protein was identified as the C-terminal fragment of E. coli protein YJAI, and was shown to contain two zinc-binding motifs. Metal-affinity chromatography and 65Zn blotting assay confirmed the ability of the 12-kDa protein to bind zinc specifically (zinc > cobalt >> cadmium). We propose that YJAI is an important component of the zinc-balancing mechanism in E. coli, the up-regulation of which with PMTR results in an increased tolerance to zinc.