ISOLATION AND CHARACTERIZATION OF PHOTOSYSTEM-I FROM 2 STRAINS OF THEMARINE OXYCHLOROBACTERIUM PROCHLOROCOCCUS

Photosystem I (PS I) complexes from two strains of the marine photosyn thetic prokaryote Prochlorococcus, MED4 (= clone CCMP1378) and SS120 ( = clone CCMP1375), were isolated by centrifugation on sucrose gradient s after detergent treatment. The PS I-enriched fractions of both strai ns contained about 100 chlorophyll molecules per P700. Electron micros copy showed that the PS I complexes were in a trimeric form. The chara cteristic long wavelength fluorescence emission of PS I at 77 K, curre ntly observed in chloroplasts and most cyanobacteria was absent both i n intact cells and in PS I preparations of both strains. The major pro teins of the PS I-enriched fractions were identified immunologically a s PsaA and PsaB. Two proteins with apparent molecular masses of about 21 and 25 kDa were present in PS I preparations of Prochlorococcus, wh ereas the small PS I subunits in cyanobacteria all have molecular mass es below 18 kDa. The 25 kDa protein showed a strong cross-reaction wit h a heterologous antibody against PsaL. Relatedness of the 21 kDa prot ein to PsaF was demonstrated by internal protein sequencing. Although only trace amounts of the major divinyl-Chl a/b-binding antenna comple xes were present in the PS I preparations, significant amounts of divi nyl-Chl b were observed in this fraction. The putative organization of this Chi b in PS I is discussed.