The structure of Photosystem I from the thermophilic cyanobacterium Synechococcus sp. determined by electron microscopy of two-dimensional crystals.
The structure of the Photosystem I (PS I) complex from the thermophilic cyanobacterium Synechococcus sp. has been investigated by electron microscopy and image analysis of two-dimensional crystals. Crystals were obtained from isolated PS I by removal of detergents with Bio-Beads. After negative staining, either single layers or two superimposed layers with a rotational different orientation were observed. The layers have a rectangular unit cell of 16.0 x 15.0 nm, which contains two PS I monomers. The monomers are arranged alternating up and down in each layer. For double-layer crystals, the images of the two layers could be separately processed by a combination of Fourier-peak-filtering and correlation averaging. Features in the two-dimensional plane can be seen with a resolution up to 1.5-1.8 nm. A model for the PS I structure was obtained by combining three-dimensional reconstructions from three tilt-series. The model shows an asymmetric PS I complex. On one side (presumably the stromal side) there is a 3 nm high ridge. This is most likely comprised of the psaC, psaD and psaE subunits. The other side (presumably the lumenal side) is rather flat, but in the center there is a 3 nm deep indentation, which possibly separates partly the two large subunits psaA and psaB.
Böttcher
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B
BIOSON Research Institute, University of Groningen, Netherlands.
Gräber
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P
Boekema
E J
EJ
D003460:Crystallization; D000458:Cyanobacteria / Q000737:chemistry*; D004591:Electrophoresis, Polyacrylamide Gel; D007091:Image Processing, Computer-Assisted; D008854:Microscopy, Electron; D008958:Models, Molecular; D045322:Photosynthetic Reaction Center Complex Proteins / Q000648:ultrastructure*; D045331:Photosystem I Protein Complex
D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex