title: SmtB is a metal-dependent repressor of the cyanobacterial metallothionein gene smtA: identification of a Zn inhibited DNA-protein complex.
abstract: The smt locus of Synechococcus PCC 7942 contains a metal-regulated gene (smtA), which encodes a class II metallothionein, and a divergently transcribed gene, smtB, which encodes a repressor of smtA transcription. Regions containing cis-acting elements required for efficient induction, and required for smtB-dependent repression, of the smtA operator-promoter were identified. Specific interactions between proteins extracted from Synechococcus PCC 7942 and defined regions surrounding the smtA operator-promoter were detected by electrophoretic mobility shift assays. Three metallothionein operator-promoter associated complexes were identified, one of which (MAC1) showed Zn-dependent dissociation and involved a region of DNA immediately upstream of smtA. Treatment with Zn-chelators facilitated re-association of MAC1 in vitro. MAC1 was not observed in extracts from smt deficient mutants but was restored in extracts from mutants complemented with a plasmid borne smtB. SmtB is thus required for the formation of a Zn-responsive complex with the smt operator-promoter and based upon the predicted structure of SmtB we propose direct SmtB-DNA interaction exerting metal-ion inducible negative control.
authors.affiliation: Department of Biological Sciences, University of Durham, UK.
authors.forename: A P
authors.initials: AP
authors.lastname: Morby
authors.forename: J S
authors.initials: JS
authors.lastname: Turner
authors.forename: J W
authors.initials: JW
authors.lastname: Huckle
authors.forename: N J
authors.initials: NJ
authors.lastname: Robinson
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000378:metabolism*; D001483:Base Sequence; D000458:Cyanobacteria; D004268:DNA-Binding Proteins / Q000502:physiology*; D008668:Metallothionein / Q000235:genetics*; D008969:Molecular Sequence Data; D009875:Operator Regions, Genetic / Q000502:physiology; D011401:Promoter Regions, Genetic / Q000502:physiology; D012097:Repressor Proteins / Q000502:physiology*; D017384:Sequence Deletion; D017386:Sequence Homology, Amino Acid; D015032:Zinc / Q000502:physiology; D001616:beta-Galactosidase / Q000235:genetics
chemical_list: D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D012097:Repressor Proteins; D008668:Metallothionein; D001616:beta-Galactosidase; D015032:Zinc