Robert C. Ford

Crystallization of the photosystem I reaction centre.

The reaction centre of the photosynthetic membrane complex photosystem I (PSI) from the thermophilic cyanobacterium Phormidium laminosum was found to crystallize under a range of conditions. The crystallization method, which can occur in the presence of larger detergent molecules than those used previously for the crystallization of membrane proteins, is presented in this report. Several crystal forms have been observed, and some of these show birefringence and linear dichroism. Optical measurements on crystals thicker than ˜5 µm were severely restricted because of the very high chlorophyll density within the crystals, but linear dichroism measurements on thin single crystals were possible and the results are presented here. By comparing the data with earlier measurements on oriented PSI complexes, a working model for the orientation of the PSI complexes within the crystal could be proposed. The PSI reaction centre is one of the largest and most complex membrane protein units that have been crystallized to date.

Structural studies on improved crystals of the photosystem I reaction centre from Phormidium laminosum

Improved crystallization methods led to the growth of large single crystals of the photosystem I reaction centre isolated from the thermophilic cyanobacterium Phormidium laminosum. Although the crystals exhibited a high degree of mosaicity, some preliminary X‐ray diffraction studies were performed. The highest order reflections found correspond to interplanar spacings of approx. 0.8 nm. In conjunction with electron microscopy of thin three‐dimensional crystals, a triclinic or monoclinic crystal system with unit cell dimensions 30 × 18 × 18 nm was assigned. In addition, our data suggest the presence of four trimeric complexes per unit cell, which may be organised as two face‐to‐face pairs.