title: Zn2+-sensing by the cyanobacterial metallothionein repressor SmtB: different motifs mediate metal-induced protein-DNA dissociation.
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+].
authors.affiliation: Department of Biochemistry and Genetics, The Medical School, University of Newcastle, UK.
authors.forename: J S
authors.initials: JS
authors.lastname: Turner
authors.forename: P D
authors.initials: PD
authors.lastname: Glands
authors.forename: A C
authors.initials: AC
authors.lastname: Samson
authors.forename: N J
authors.initials: NJ
authors.lastname: Robinson
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins; D003001:Cloning, Molecular; D000458:Cyanobacteria / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics; D008668:Metallothionein / Q000235:genetics*; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009875:Operator Regions, Genetic; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D015032:Zinc / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D011994:Recombinant Proteins; D012097:Repressor Proteins; D008668:Metallothionein; D015032:Zinc