Monomeric and trimeric forms of photosystem I reaction center of Mastigocladus laminosus: Crystallization and preliminary characterization

Photosystem I (PSI) reaction centers (RCs) of the thermophilic cyanobacterium Mastigocladus laminosus were purified and characterized. The PSI RC was obtained in two forms, monomeric and trimeric. The two forms contained the same number of pigments per P₇₀₀ and displayed similar photochemical activities. The two forms had nearly identical polypeptide subunit compositions; the only observed difference was an additional subunit of about 12 kDa observed in the trimeric form. The purified preparations of both the monomeric and the trimeric forms were used for crystallization and preliminary cristallographic analysis. The trimeric PSI RC preparations produced several three-dimensional crystal forms, one of which, the "hexagonal needle" form (T_HN), had a hexagonal unit cell with dimensions of 300 × 300 × 160 Å, containing four PSI RC trimers. The monomeric preparations also produced single crystals of several forms under various crystallization conditions. One of these crystal forms, the "hexagonal plate" (M_HP), diffracted to a resolution of about 5.5 Å. It had a hexagonal unit cell with dimensions of 192 × 192 × 163 Å, containing six PSI RC monomers. Comparison of the PSI RCs in the crystals with those in the precrystallization preparations demonstrated that neither the monomeric nor the trimeric form of PSI RC was altered by the crystallization process. Both forms retained their original polypeptide subunit composition and their pigment content.