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

Large, well-ordered two-dimensional crystals of the histidine-tagged-HupR p rotein, a transcriptional regulator from the photosynthetic bacterium Rhodo bacter capsulatus, were obtained by specific interaction with a Ni2+-chelat ed lipid monolayer. HupR is a response regulator of the NtrC subfamily; it activates the transcription of the structural genes hupSLC, of [NiFe]hydrog enase. A projection map of the full-length protein at 9 Angstrom resolution was obtained by electron cryo-microscopy and image analysis of frozen-hydr ated two-dimensional crystals. The crystals have a plane group with unit ce ll dimensions of a=b=111.6(+/-1.0) gamma=120.4(+/-0.5)degrees. The structur e of the N-terminal domain of NtrC, the family to which HupR belongs, had b een determined previously by NMR. The atomic coordinates of the N-terminal domain of NtrC, were compared to the structure obtained by cryo-electron mi croscope techniques of the whole HupR. These results provide the first stru cture at medium resolution of a whole transcription factor, HupR from the N trC family. (C) 2000 Academic Press.