An SmtB-like repressor from Synechocystis PCC 6803 regulates a zinc exporter

ORF slr0798, now designated ziaA , from Synechocystis PCC 6803 encodes a polypeptide with sequence features of heavy metal transporting P-type ATPases. Increased Zn ²⁺ tolerance and reduced ⁶⁵ Zn accumulation was observed in Synechococcus PCC 7942, strain R2-PIM8( smt ), containing ziaA and upstream regulatory sequences, compared with control cells. Conversely, reduced Zn ²⁺ tolerance was observed following disruption of ziaA in Synechocystis PCC 6803, and ziaA -mediated restoration of Zn ²⁺ tolerance has subsequently been used as a selectable marker for transformation. Nucleotide sequences upstream of ziaA , fused to a promoterless lacZ gene, conferred Zn ²⁺ -dependent β-galactosidase activity when introduced into R2-PIM8( smt ). The product of ORF sll0792, designated ZiaR, is a Zn ²⁺ -responsive repressor of ziaA transcription. Reporter gene constructs lacking ziaR conferred elevated Zn ²⁺ -independent expression from the ziaA operator–promoter in R2-PIM8( smt ). Gel retardation assays detected ZiaR-dependent complexes forming with the zia operator–promoter and ZiaR–DNA binding was enhanced by treatment with a metal-chelator in vitro . Two mutants of ZiaR (C71S/C73S and H116R) bound to, and repressed expression from, the ziaA operator–promoter but were unable to sense Zn ²⁺ . Metal coordination to His-imidazole and Cys-thiolate ligands at these residues of ZiaR is thus implicated in Zn ²⁺ -perception by Synechocystis PCC 6803.