In response to an increased level of Zn
2+,
Synechococcus sp. PCC 7942 expresses SmtA, a metallothionein-like metal-chelating protein, while
Synechocystis sp. PCC 6803 expresses ZiaA, a transporter of Zn
2+. 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 Zn
2+, 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 Zn
2+ at higher concentrations than 16
mM. On the other hand, the corresponding Zn
2+ 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 Zn
2+ on the dissociation of these complexes are mainly associated with the structures of the repressors.
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