Zinc ions inhibit the protein-DNA complex formation between cyanobacterial transcription factor SmtB and its recognition DNA sequences

SmtB is a trans-acting dimeric repressor of the metal-regulated smtA gene, and the release of SmtB from the smtA operator/promoter region is essential for the tolerance to Zn2+ stress by SmtA expression. Gel retardation assaying demonstrated that different sizes of SmtB–DNA complexes were formed depending on the DNA sequences, and the amounts of these complexes decreased in the presence of Zn2+. Here, we present the first direct evidence that Zn2+ (>4 µM) inhibits the SmtB–DNA complex formation in vitro, which ensures the physiological functions of SmtB as a Zn2+ sensor and a transcription factor.