Abstract
We improved the resolution of PSII crystals from a thermophilic, acidophilic red alga Cyanidium caldarium to a 3.5 angstrom resolution by means of improvement of crystallization conditions and post-crystallization dehydration. The dehydration conditions were found to be most important for improving the crystal resolution, and we optimized these conditions to obtain the highest resolution data. Based on the X-ray diffraction data obtained, we analyzed the molecular packing of PSII within the crystals using the molecular replacement method with PSII dimer structure of cyanobacteria as the searching model. The result showed that there were 2 PSII dimers in each asymmetric unit in the red algal crystal, which gives rise to 16 PSII monomers per unit cell in the red algal crystal. This significantly contrasts to 8 PSII monomers per unit cell in the cyanobacterial crystal.
