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J. Bacteriol. doi:10.1128/JB.01531-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Ligand-Controlled Proteolysis of the Escherichia coli Transcriptional Regulator ZntR

Howard Hughes Medical Institute, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139; Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

^* To whom correspondence should be addressed. Email: tabaker{at}mit.edu.

	Abstract

Proteases play a crucial role in remodeling the bacterial proteome in response to changes in cellular environment. Escherichia coli ZntR, a zinc-responsive transcriptional regulator, was identified by proteomic experiments as a likely ClpXP substrate suggesting that protein turnover may play a role in regulation of zinc homeostasis. When intracellular zinc levels are high, ZntR activates expression of ZntA, an ATPase essential for zinc export. We find that ZntR is degraded in vivo in a manner dependent on both the ClpXP and Lon proteases. However, ZntR degradation decreases in the presence of high zinc concentrations, the level of ZntR rises, and transcription of the zntA exporter is increased. Mutagenesis experiments reveal that zinc binding does not appear to be solely responsible for the zinc-induced protection from proteolysis. Therefore we tested whether DNA binding was important in the zinc-induced stabilization of ZntR by mutagenesis of the DNA binding helices. Replacement of a conserved arginine (R19A) in the DNA-binding domain both enhances ZntR degradation and abolishes zinc-induced transcriptional activation of zntA. Biochemical and physical analysis of ZntR^R19A demonstrate that it is structurally similar to, and binds zinc as well as the wild-type protein, but is severely defective in binding DNA. Thus, we conclude that two different ligands - zinc and DNA - function together to increase ZntR stability and that ligand-controlled proteolysis of ZntR plays an important role in fine-tuning zinc homeostasis in bacteria.

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