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.