Requirement of nitric oxide for induction of genes whose products are involved in nitric oxide metabolism in Rhodobacter sphaeroides 2.4.3.

During denitrification, freely diffusible nitric oxide (NO) is generated for use as a terminal electron acceptor. NO is produced by nitrite reductase (Nir) and reduced to nitrous oxide by nitric oxide reductase (Nor). Using Nir and Nor-deficient mutants of Rhodobacter sphaeroides 2.4.3, we have shown that the endogenous production of NO or the addition of exogenous NO induces transcription of certain genes encoding Nir and Nor. A Nor-deficient strain was found to be capable of expressing wild type levels of nirK-lacZ and norB-lacZ fusions in medium unamended with nitrogen oxides. When this experiment is performed in the presence of hemoglobin, fusion expression is eliminated. NO and the NO-generator, sodium nitroprusside, can induce expression of both fusions in a strain lacking Nir and the consequent ability to produce NO. Sodium nitroprusside cannot induce expression of nirK-lacZ in a strain lacking the transcriptional activator NnrR (nitrite and nitric oxide reductase regulator). Addition of the cyclic nucleotides cAMP and 8-bromoguanosine-cGMP does not result in expression of either fusion. These results demonstrate that denitrifying bacteria produce NO as a signal molecule to activate expression of the genes encoding proteins required for NO metabolism.