The QseBC two-component system is one of the quorum-sensing systems in bacteria, which are involved in the regulation of flagella, motility, and transcription factors related to bacterial virulence. The sensory domain of QseC in enterohemorrhagic Escherichia coli (EHEC) O157:H7 has important roles in quorum sensing via the detection of autoinducer-3 and the regulation of pathogenesis through the recognition of host hormones, such as epinephrine and norepinephrine. Therefore, structural studies of the sensory domain of QseC could be important to understand not only quorum sensing mechanism in bacteria but also pathogenic mechanisms in host cells. Herein, the structural properties of the sensory domain of QseC from EHEC 0157:H7 in solution states are described. The findings suggest that its folding state is highly dependent on Mg2+: the protein is less structured in the absence of Mg2+ though is well structured in the presence of 120 mM Mg2+. According to multi-angle light scattering combined with size-exclusion chromatography, the protein exists as a monomer in the presence of 120 mM Mg2+. Moreover, we observed the samebehavior for the sensory domain of QseC from E. coli k12. We suggest that our results may provide useful information for the determination of the structure of the sensory domain of QseC. (C) 2014 Elsevier Ltd. All rights reserved.