Projection structure of a transcriptional regulator, HupR, determined by electron cryo-microscopy

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 Ni2+-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 Angstrom resolution was obtained by electron cryo-microscopy and image analysis of frozen-hydrated two-dimensional crystals. The crystals have a plane group with unit cell dimensions of a=b=111.6(+/-1.0) 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. (C) 2000 Academic Press.