| Shrnutí: | Large, well-ordered two-dimensional crystals of the histidine-tagged-HupR protein, a transcriptional regulator from the photosynthetic bacterium Rhodobacter capsulatus, were obtained by specific interaction with a Ni(2+)-chelated lipid monolayer. HupR is a response regulator of the NtrC subfamily; it activates the transcription of the structural genes hupSLC, of [NiFe]hydrogenase. A projection map of the full-length protein at 9 A resolution was obtained by electron cryo-microscopy and image analysis of frozen-hydrated two-dimensional crystals. The crystals have a p6 plane group with unit cell dimensions of a=b=111.6(+/-1.0) A, gamma=120.4(+/-0.5) degrees. The structure of the N-terminal domain of NtrC, the family to which HupR belongs, had been determined previously by NMR. The atomic coordinates of the N-terminal domain of NtrC, were compared to the structure obtained by cryo-electron microscope techniques of the whole HupR. These results provide the first structure at medium resolution of a whole transcription factor, HupR from the NtrC family.
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