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. 1996 Oct 1;24(19):3714–3721. doi: 10.1093/nar/24.19.3714

Zn2+-sensing by the cyanobacterial metallothionein repressor SmtB: different motifs mediate metal-induced protein-DNA dissociation.

J S Turner 1, P D Glands 1, A C Samson 1, N J Robinson 1
PMCID: PMC146171  PMID: 8871549

Abstract

SmtB is a member of a family of repressors which dissociate from DNA in the presence of metals; Zn2+ being the most potent inducer of metallothionein gene (smtA) transcription in vivo. In Synechococcus PCC 7942 cells devoid of chromosomal smtB, four plasmid-encoded mutants of SmtB (C61S, T11S/C14S, C121S and H105R/H106R) repressed lacZ expression driven by the smtA operator-promoter. Gel retardation assays with extracts from the complemented cells detected multiple SmtB-dependent complexes similar to those obtained with extracts from wild-type cells or with recombinant-SmtB. Elevated [Zn2+] alleviated repression in vivo by all of the mutants except H105R/H106R. These His residues (one or both) are therefore essential for Zn2+-sensing while, contrary to expectations, Cys residues are not. Hence different motifs facilitate metal-induced DNA-dissociation by SmtB and ArsR (the related oxyanion-sensing repressor), presumably generating variety in the spectra of metals sensed. Nucleotides and amino acids involved in DNA-SmtB interaction have been further defined/inferred and we also confirm that additional unknown factors form specific associations with the smt operator-promoter in elevated [Zn2+].

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Selected References

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