Abstract
In Synechococcus sp. PCC 7942, expression of smtA gene encoding a metallothionein-like protein, SmtA, is regulated by a transcription factor, SmtB. Excess amount of Zn2+ cause the dissociation of SmtB from the operator/promoter region (o/p) of smtA, resulting in the initiation of smtA transcription. In another cyanobacterium, Synechococcus sp. PCC 7002, homologues of SmtA and SmtB appeared to be present from the genome data base. However, the 7002-SmtA is expressed at higher concentrations of Zn2+ than the 7942-SmtA, indicating the difference mechanism in response to Zn2+. The gel-retardation assay showed that 7942-SmtB formed 4 different complexes with the 7942-o/p, whereas 7002-SmtB formed 6 with the 7002-o/p. It is also found concentrations of Zn2+ that inhibits formations of protein-DNA complexes are different between 7942-SmtB and 7002-SmtB. These results suggest that the structural difference of SmtB as well as o/p region are related to specific response to Zn2+ in two cyanobacteria.
