Two redox-responsive transcription regulators have been well defined in Esc
herichia coli and serve as paradigms of redox-operated genetic switches. So
xR contains iron-sulfur centers that activate the protein when they are one
-electron oxidized, or nitrosylated by nitric oxide. OxyR contains a pair o
f redox-active cysteine residues that activate the protein when they are ox
idized to form a disulfide bond.