Successful respiration in Bacillus subtilis using oxygen or nitrate as the terminal electron acceptor requires the ResD-ResE signal transduction system. Although transcription of ResDE-controlled genes is induced at the stationary phase of aerobic growth, it is induced to a higher extent upon oxygen limitation. Furthermore, maximal transcriptional activation requires not only oxygen limitation, but also nitric oxide (NO). Oxygen limitation likely results in conversion of the ResE sensor kinase activity from a phosphatase-dominant to a kinase-dominant mode. In addition, low oxygen levels promote the production and maintenance of NO during nitrate respiration, which leads to elimination of the repression exerted by the NO-sensitive transcriptional regulator NsrR. ResD, after undergoing ResE-mediated phosphorylation, interacts with the C-terminal domain of the α subunit of RNA polymerase to activate transcription initiation at ResDE-controlled promoters.