The negative transcriptional regulator NmrA discriminates between oxidized and reduced dinucleotides

NmrA, a transcription repressor involved in the regulation of nitrogen metabolism in Aspergillus nidulans, is a member of the short-chain dehydrogenase reductase superfamily. Isothermal titration calorimetry and differential scanning calorimetry have been used to show NmrA binds NAD(+) and NADP(+) with similar affinity ( average K-D 65 muM) but has a greatly reduced affinity for NADH and NADPH (average K-D 6.0 mM). The structure of NmrA in a complex with NADP(+) reveals how repositioning a His-37 side chain allows the different conformations of NAD(+) and NADP(+) to be accommodated. Modeling NAD(P) H into NmrA indicated that steric clashes, attenuation of electrostatic interactions, and loss of aromatic ring stacking can explain the differing affinities of NAD(P)(+)/NAD(P) H. The ability of NmrA to discriminate between the oxidized and reduced forms of the dinucleotides may be linked to a possible role in redox sensing. Isothermal titration calorimetry demonstrated that NmrA and a C-terminal fragment of the GATA transcription factor AreA interacted with a 1: 1 stoichiometry and an apparent K-D of 0.26 muM. NmrA was unable to bind the nitrogen metabolite repression signaling molecules ammonium or glutamine.