Structural characterisation of the photosystem two reaction centre from Synechococcus elongatus using electron microscopy and single particle analysis.

Abstract

Here we present a 3D map of a photosystem II (PSII) dimeric core complex from the thermophilic cyanobacterium Synechococcus elongatus by electron cryo-microscopy and single particle analysis. The resolution was calculated as 15 Å (Fourier Shell Correlation 3s *Ö 2), ideal for comparing the 30Å model from negatively stained PSII dimer single particles (Nield et al. (2000) Journal of Biological Chemistry, 275, 27940-27946) and the more recent 3.8 Å model determined by X-ray diffraction (Zouni et al. (2001) Nature, 409, 739-743). We have been able to confidently fit the 3D co-ordinates from the X-ray diffraction model into specific densities within our 15 Å 3D map. Furthermore, in order to localise densities attributed to the PsbO, PsbU and PsbV lumenal extrinsic proteins of PSII, a number of different 3D maps were determined from biochemically treated preparations, which were then visualised in negative stain. Selective washing experiments yielded a PSII core homodimer without any lumenal extrinsic proteins, and another core containing just the PsbU and PsbV. In addition, by comparing the 15 Å vitrified specimen-derived model and the 3D maps from negatively stained particles, difference mapping allowed for confirmation of the position of the 33 kDa extrinsic protein (PsbO) relative to the underlying transmembrane helices. The models presented here were also compared with 3D maps obtained locally from higher plant PSII and the cyanobacterium Synechocystis PCC 6803.