The ResD–ResE two‑component system regulates anaerobic nitrate respiration in Bacillus subtilis. ResE phosphorylates ResD, enabling activation of fnr, nasDEF, and hmp. Footprinting analyses show that ResD binds a conserved TTGTAAN₃TTN₄A motif as flexible monomers or dimers. Activation of fnr and nasD requires specific residues in the RNA polymerase α subunit, while hmp activation involves σA. Unphosphorylated ResD also activates target genes under oxygen limitation, with NsrR modulating expression in response to nitric oxide.
