Abstract
In response to an increased level of Zn2+, Synechococcus sp. PCC 7942 expresses SmtA, a metallothionein-like metal-chelating protein, while Synechocystis sp. PCC 6803 expresses ZiaA, a transporter of Zn2+. The gene expression of these proteins is regulated by repressor protein, SmtB and ZiaR, respectively. In spite of contributing to different response systems, both repressor proteins belong to the ArsR family and are highly homologous to each other. To understand the different systems responsible for dealing with excess Zn2+, we examined the cis-elements in the promoter regions of smtA and ziaA, as well as the binding affinities of recombinant SmtB and ZiaR proteins. The operator/promoter region of smtA included two palindromic sequences and that of ziaA included one. Electrophoretic mobility shift assay revealed that SmtB formed four different complexes with the operator/promoter region of smtA, whereas it formed only two different complexes with the corresponding region of ziaA. For ZiaR, the corresponding results were quite the same as those for SmtB. Furthermore, the complex formation between SmtB and operator/promoter regions is inhibited in the presence of Zn2+ at higher concentrations than 16 mM. On the other hand, the corresponding Zn2+ concentration is 128 mM. These results demonstrate that the degrees of protein-DNA complex formation between repressor proteins and the operator/promoter regions of regulated genes depend on the structures of the operator/promoter regions, and the effects of Zn2+ on the dissociation of these complexes are mainly associated with the structures of the repressors.
