Isolation and characterization of Photosystem I from two strains of the marine oxychlorobacterium Prochlorococcus

Photosystem I (PS I) complexes from two strains of the marine photosynthetic prokaryote Prochlorococcus, MED4 (= clone CCMP1378) and SS120 (= clone CCMP1375), were isolated by centrifugation on sucrose gradients after detergent treatment. The PS I-enriched fractions of both strains contained about 100 chlorophyll molecules per P700. Electron microscopy showed that the PS I complexes were in a trimeric form. The characteristic long wavelength fluorescence emission of PS I at 77 K, currently observed in chloroplasts and most cyanobacteria was absent both in intact cells and in PS I preparations of both strains. The major proteins of the PS I-enriched fractions were identified immunologically as PsaA and PsaB. Two proteins with apparent molecular masses of about 21 and 25 kDa were present in PS I preparations of Prochlorococcus, whereas the small PS I subunits in cyanobacteria all have molecular masses below 18 kDa. The 25 kDa protein showed a strong cross-reaction with a heterologous antibody against PsaL. Relatedness of the 21 kDa protein to PsaF was demonstrated by internal protein sequencing. Although only trace amounts of the major divinyl-Chl a/b-binding antenna complexes were present in the PS I preparations, significant amounts of divinyl-Chl b were observed in this fraction. The putative organization of this Chl b in PS I is discussed.