Interacting selectively and non-covalently with a specific DNA sequence and interacting selectively and non-covalently with any metal ion(s) and responding, e.g. by conformational change, to changes in the cellular level of those metal ions, in such a way that metal ion binding regulates the activity of the transcription factor in order to modulate transcription by an RNA polymerase. Specific mechanisms include increasing or decreasing the ability of the transcription factor to interact selectively and non-covalently with a specific DNA sequence (for example S. cerevisiae Cup2p or the prokaryotic SmtB/ArsR family, changing the conformation of transcription factor bound DNA between the apo- and metal-bound forms of the transcription factor. Additional mechanisms may exist.