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

The sigma54-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 Cys(112) 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.