The Rex-family repressors sense redox levels by alternative binding to NADH or NAD(+). RSP is the homologue of Rex in Thermoanaerobacter ethanolicus JW200(T) and regulates ethanol fermentation in this obligate anaerobe. The dimeric repressor binds to DNA by an open conformation. The crystal structure of RSP/alpha-NAD(+) complex shows a different set of ligand interactions mainly due to the unique configuration of the nicotinamide moiety. The positively charged ring is covered by the Tyr102 side chain and interacts with a sulfate ion adjacent to the N-terminus of helix alpha 8. Consequently, the RSP dimer may be locked in a closed conformation that does not bind to DNA. However, alpha-NAD(+) does not show a higher affinity to RSP than B-NAD+. It has to be improved for possible use as an effector in modulating the repressor. (C) 2014 Elsevier Inc. All rights reserved.