Studies on the protein-DNA complex formation between the cyanobacterial transcription factors, SmtB and its homologues, functioning as zinc-ion sensors and the recognition DNA sequences.

Abstract

In cyanobacterium Synechococcus sp. PCC 7942, SmtB, functioning as the sensor to heavy-metal ions (notably Zn-ion) in the dimer form, represses the transcription of smtA gene encoding metallothionein-like protein. There are two recognition DNA sequences in the operator/promoter region of smtA gene, and the binding of SmtB with Zn-ions reduces the affinities to these sequences. This induces the activation of smtA transcription. In this study, we have studied the functional differences between these two recognition DNA sequences, with spectroscopic (hetero-nuclear NMR and UV-resonance Raman) and biochemical methods. On the basis of the results obtained here, we clarified the regulation mechanism of the smtA expression. Similar regulatory system for the tolerance to Zn-ion stress is found in another cyanobacterium. We also found that the functionally important differences between these two systems are mainly due to the structural differences of recognition DNA sequences.