title: Isolation of a prokaryotic metallothionein locus and analysis of transcriptional control by trace metal ions.
abstract: In eukaryotes, metallothioneins (MTs) are involved in cellular responses to elevated concentrations of certain metal ions. We report the isolation and analysis of a prokaryotic MT locus from Synechococcus PCC 7942. The MT locus (smt) includes smtA, which encodes a class II MT, and a divergently transcribed gene, smtB. The sites of transcription initiation of both genes have been mapped and features within the smt operator-promoter region identified. Elevated concentrations of the ionic species of Cd, Co, Cr, Cu, Hg, Ni, Pb and Zn elicited an increase in the abundance of smtA transcripts. There was no detectable effect of elevated metal (Cd) on smtA transcript stability. Sequences upstream of smtA, fused to a promoterless lacZ gene, conferred metal-dependent beta-galactosidase activity in Synechococcus PCC 7942 (strain R2-PIM8). At maximum permissive concentrations, Zn was the most potent elicitor in vivo, followed by Cu and Cd with slight induction by Co and Ni. The deduced SmtB polypeptide has similarity to the ArsR and CadC proteins involved in resistance to arsenate/arsenite/antimonite and to Cd, contains a predicted helix-turn-helix DNA-binding motif and is shown to be a repressor of transcription from the smtA operator-promoter.
authors.affiliation: Department of Biological Sciences, University of Durham, UK.
authors.forename: J W
authors.initials: JW
authors.lastname: Huckle
authors.forename: A P
authors.initials: AP
authors.lastname: Morby
authors.forename: J S
authors.initials: JS
authors.lastname: Turner
authors.forename: N J
authors.initials: NJ
authors.lastname: Robinson
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000096:biosynthesis / Q000235:genetics*; D001483:Base Sequence; D002412:Cations / Q000494:pharmacology*; D016384:Consensus Sequence; D000458:Cyanobacteria / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000096:biosynthesis / Q000235:genetics* / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial / Q000187:drug effects*; D005798:Genes, Bacterial; D008668:Metallothionein / Q000096:biosynthesis / Q000235:genetics*; D008969:Molecular Sequence Data; D011387:Prokaryotic Cells / Q000187:drug effects / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis; D012097:Repressor Proteins / Q000096:biosynthesis / Q000235:genetics*; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D014131:Trace Elements / Q000494:pharmacology*; D015534:Trans-Activators / Q000235:genetics; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects*
chemical_list: D001426:Bacterial Proteins; C080050:CadC protein, Bacteria; D002412:Cations; D004268:DNA-Binding Proteins; D011993:Recombinant Fusion Proteins; D012097:Repressor Proteins; D014131:Trace Elements; D015534:Trans-Activators; D014157:Transcription Factors; C080048:smtA protein, Synechococcus; D008668:Metallothionein