An electron microscopic study of the archaeal feast/famine regulatory protein: 7. Single particles formed by pot0434017

Particles formed by a feast/famine regulatory protein (FFRP), pot0434017 (FL11), in solution in the absence of DNA were analyzed using electron microscopy (EM). By applying conventional (i.e. dry) EM to the protein negatively stained with uranyl acetate, top views of tetrameric assemblies of dimers were obtained, where four pairs each of N-domains were extending from C-domains assembled around the centers. In cryo-EM images of the protein embedded in 3D amorphous ice, sets of four densities were arranged around ellipsoids having similar lengths for their long axes but of different lengths for their short axes. These images were interpreted as projections with different tilts of four pairs of N-domains arranged inside flat assemblies: the positively charged N-domains only were stained with ammonium molybdate, but the negatively charged C-domains were unstained and thus unobservable. Using seventeen such cryo-images, in combination with a crystal structure equivalent to an assembly of C-domains, a disk-like 3D structure was reconstructed.