NO-dependent transcriptional activation of gene expression in Ralstonia eutropha H16

The σ54-dependent transcriptional regulator NorR of Ralstonia eutropha H16 activates gene expression in response to nitric oxide (NO). The N-terminal domain of NorR is thought to be involved in signal perception. A C112S exchange within this domain abolished promoter activation by the mutated protein, indicating that Cys¹¹² is essential for the signalling mechanism of NorR. The DNA region recognized by NorR contains three copies of a conserved element termed the NorR-box. Alteration of bases within any of the NorR-boxes resulted in a significant decrease in promoter activation. Therefore all three boxes have to be recognized by NorR to activate its target promoter. NorR controls expression of an operon that encodes a redox-active non-haem-iron protein NorA and an NO reductase NorB. NorA exerts a negative effect on signal-dependent promoter activation by NorR. Optical spectroscopy of purified NorA indicates that the reduced protein can react with NO to form a ferrous nitrosyl adduct. Hence, NO binding by NorA opens up the possibility that NorA and NorR compete for NO in the cytoplasm.