Expression, purification, crystallization and preliminary X-ray analysis of the DNA-binding domain of a Chlamydia trachomatis OmpR/PhoB-subfamily response regulator homolog, ChxR.

Two-component signal transduction systems in bacteria are a primary mechanism for responding to environmental stimuli and adjusting gene expression accordingly. Generally in these systems a sensor kinase phosphorylates a response regulator that regulates transcription. Response regulators contain two domains: a receiver domain and an effector domain. The receiver domain is typically phosphorylated and as a result facilitates the DNA-binding and transcriptional activity of the effector domain. The OmpR/PhoB subfamily is the largest of the response-regulator subfamilies and is primarily defined by the winged helix-turn-helix DNA-binding motif within the effector domain. The overall structure of effector domains is highly conserved and contains three defined elements that are critical for transcriptional regulation: a DNA major-groove binding helix, a DNA minor-groove binding wing and a transcriptional activation loop. These functional elements are often diverse in sequence and conformation and reflect the functional differences observed between individual subfamily members. ChxR from Chlamydia trachomatis is an atypical OmpR/PhoB response regulator homolog that has transcriptional activity in the absence of phosphorylation. To facilitate the precise identification of the functional elements of the ChxR effector domain, this protein was cloned, expressed, purified and crystallized. Crystals were obtained from two separate mother liquors, producing two morphologically distinct crystals. The space group of both crystals was P4(3)2(1)2 (or its enantiomorph P4(1)2(1)2) with isomorphous unit-cell parameters; the crystals diffracted to 2.2-2.5 A resolution.