AzuR From the SmtB/ArsR Family of Transcriptional Repressors Regulates Metallothionein in Anabaena sp. Strain PCC 7120

Abstract

Metallothioneins (MTs) are cysteine-rich, metal-sequestering cytosolic proteins that play a key role in maintaining metal homeostasis and detoxification. We had previously characterized NmtA, a MT from the heterocystous, nitrogen-fixing cyanobacterium Anabaena sp. strain PCC 7120 and demonstrated its role in providing protection against cadmium toxicity. In this study, we illustrate the regulation of Anabaena NmtA by AzuR (Alr0831) belonging to the SmtB/ArsR family of transcriptional repressors. There is currently no experimental evidence for any functional role of AzuR. It is observed that azuR is located within the znuABC operon but in the opposite orientation and remotely away from the nmtA locus. Sequence analysis of AzuR revealed a high degree of sequence identity with Synechococcus SmtB and a distinct α5 metal binding site similar to that of SmtB. In order to characterize AzuR, we overexpressed it in Escherichia coli and purified it by chitin affinity chromatography. Far-UV circular dichroism spectroscopy indicated that the recombinant AzuR protein possessed a properly folded structure. Glutaraldehyde cross-linking and size-exclusion chromatography revealed that AzuR exists as a dimer of ∼28 kDa in solution. Analysis of its putative promoter region [100 bp upstream of nmtA open reading frame (ORF)] identified the presence of a 12–2–12 imperfect inverted repeat as the cis-acting element important for repressor binding. Electrophoretic mobility shift assays (EMSAs) showed concentration-dependent binding of recombinant dimeric AzuR with the promoter indicating that NmtA is indeed a regulatory target of AzuR. Binding of AzuR to DNA was disrupted in the presence of metal ions like Zn2+, Cd2+, Cu2+, Co2+, Ni2+, Pb2+, and Mn2+. The metal-dependent dissociation of protein–DNA complexes suggested the negative regulation of metal-inducible nmtA expression by AzuR. Overexpression of azuR in its native strain Anabaena 7120 enhanced the susceptibility to cadmium stress significantly. Overall, we propose a negative regulation of Anabaena MT by an α5 SmtB/ArsR metalloregulator AzuR.

Unbalancing Zur (FurB)‐mediated homeostasis in Anabaena sp. PCC7120: Consequences on metal trafficking, heterocyst development and biofilm formation

“…In addition to the set of expected DEGs previously reported as Zur/FurB‐targets (Napolitano et al, 2012), a series of novel genes involved in the homeostasis and transport of metals exhibited altered transcription in the Δ zur strain. Among them, the smtB ‐like repressor aztR ( all7621 ) and the divergently transcribed Zn/Pb exporting ATPase aztA ( alr7622 ) putatively involved in zinc homeostasis (Divya & Acharya, 2021; Napolitano et al, 2012) were upregulated. Also, a set of transporters involved in the homeostasis of other metals displayed altered transcription levels in the Δ zur strain, including the downregulation of several TonB‐dependent iron transporters.…”