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. 1993 Feb 25;21(4):921–925. doi: 10.1093/nar/21.4.921

SmtB is a metal-dependent repressor of the cyanobacterial metallothionein gene smtA: identification of a Zn inhibited DNA-protein complex.

A P Morby 1, J S Turner 1, J W Huckle 1, N J Robinson 1
PMCID: PMC309225  PMID: 8451191

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.

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Selected References

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