The sigma(54)-dependent transcription in bacteria is associated with various stress and growth conditions. Activators of the sigma(54) protein contain a central domain belonging to the AAA+ superfamily of ATPases, members of which function in diverse cellular processes in both prokaryotic and eukaryotic cells. We describe the X-ray structure of an N-terminal domain deletion of the ZraR protein from Salmonella typhimurium, which is a homologue of the general nitrogen regulatory protein NtrC, at 3 angstrom resolution. The structure reveals a hexameric ring that is typical for AAA+ containing proteins but which differs from the heptameric ring found in the crystal structure of the AAA+ domain of NtrCl from Aquifex aeolicus. The dimerisation interface between DNA-binding domains observed in the crystal structure suggests that dodecamers, rather than hexamers, might be the functionally important oligomer. (C) 2005 Elsevier Inc. All rights reserved.