The Bacillus subtilis Spx protein is a global transcription factor that interacts with the C-terminal domain of the RNA polymerase alpha subunit (alpha CTD) and regulates transcription of genes involved in thiol-oxidative stress, sporulation, competence, and organosulfur metabolism. Here we determined the X-ray crystal structure of the Spx/alpha CTD complex from an entirely new crystal form than previously reported [Newberry, K.J., Nakano, S., Zuber, P., Brennan, R.G., 2005. Crystal structure of the Bacillus subtilis anti-alpha, global transcriptional regulator, Spx, in complex with the alpha C-terminal domain of RNA polymerase. Proc. Natl. Acad. Sci. USA 102, 15839-15844]. Comparison of the previously reported sulfate-bound complex and our sulfate-free complex reveals subtle conformational changes that may be important for the role of Spx in regulating organosulfur metabolism. (C) 2009 Elsevier Inc. All rights reserved.