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

Abstract

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 Ni 2؉ , Co 2؉ , Cd 2؉ , Mn 2؉ , or Fe 2؉ , 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 zincbinding motifs. Metal-affinity chromatography and 65 Zn 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.Heavy metals play an important role in the metabolism of eukaryotic and prokaryotic cells. Zinc, copper, cobalt, and nickel are essential for functional activity and/or structural stability of a large variety of proteins (1-3), while other metals, such as mercury and lead, are toxic, and their accumulation in the cell has inhibitory effects on various cell functions (4, 5). High concentrations of essential heavy metals could also be deleterious; consequently, cells must precisely regulate their availability (6). Two basic mechanisms of heavy metal resistance have been identified: intracellular sequestration through formation of complexes with metal-specific proteins (such as phytochelatins in plants and yeast, or metallothioneins in animals, plants, yeast, and cyanobacteria), and reduced accumulation based on an active efflux of the cation (found in both eukaryotes and prokaryotes) (7). Some of the heavy metal binding and transporting proteins, including metallothioneins and copper-transporting P-type ATPases of mammals, bacteria, and yeast were remarkably preserved during evolution, suggesting that certain ways of regulation of essential microelements are very similar for eukaryotic and prokaryotic cells (8).Zinc plays a particularly important role in cell homeostasis. More than 300 known enzymes require zinc for their catalytic functions. The essential role of zinc in protein structure stabilization and folding has been illustrated by the discovery and characterization of the eukaryotic zinc finger transcription factors and the large family of hormone receptor proteins (8). Appreciation of the importance of zinc for cell metabolism has stimulated ge...

Cobalt-dependent Transcriptional Switching by a Dual-effector MerR-like Protein Regulates a Cobalt-exporting Variant CPx-type ATPase

“…6). During the course of this work, a MerR-like protein from E. coli has been shown to activate transcription from the zntA operator-promoter in response to zinc (41), and a similar activity suggested for a homologue from Proteus mirabilis (42). It will be intriguing to determine how/if responses of ZntR are modified coincident with fluctuating requirements for zinc.…”

Structure of the Periplasmic Stress Response Protein CpxP

“…ZraP is a metal-binding protein that is induced by a two-component system that responds to zinc or lead (33,42), while CnrX is thought to act as a metal sensor that binds cobalt, nickel, and potentially copper in an extracytoplasmic function (ECF)-sigma factor signaling pathway (18,19,34,38,61). Interestingly, copper ions have been implicated as an inducer of the Cpx pathway (26,(67)(68)(69).…”

“…Both CnrX and ZraP function in bacterial stress response pathways that are responsible for the efflux of toxic metals from the cell (19,33,34,42,61). ZraP is a metal-binding protein that is induced by a two-component system that responds to zinc or lead (33,42), while CnrX is thought to act as a metal sensor that binds cobalt, nickel, and potentially copper in an extracytoplasmic function (ECF)-sigma factor signaling pathway (18,19,34,38,61).…”

AztD, a Periplasmic Zinc Metallochaperone to an ATP-binding Cassette (ABC) Transporter System in Paracoccus denitrificans

“…Under severe zinc limitation, high affinity solute-binding proteins (SBPs) 3 such as ZnuA compete effectively for zinc (15), delivering it to the cytoplasm via the ATP-binding cassette (ABC) transporter system ZnuABC. Zinc stress, however, induces the expression of ZraP, a small periplasmic zincbinding protein that has been suggested to chelate excess zinc and modulate its specific efflux or prevent its import by low affinity transporters (16). The periplasmic protein ZinT has also been implicated in maintaining zinc homeostasis in the periplasm, although its precise role is still unclear.…”