A NOVEL 3.5 KDA PROTEIN-COMPONENT OF CYANOBACTERIAL PHOTOSYSTEM-I COMPLEXES

A novel protein component of 3.5 kDa was detected in photosystem I complexes prepared from several cyanobacteria, viz. Synechococcus vulcanus, Synechococcus elongatus BP-1, Synechococcus sp. PCC 7002 and Synechocystis sp. PCC 6803. The complete amino acid sequence of this component was determined by direct protein sequencing. The sequences of the 3.5 kDa proteins from these four organisms were highly homologous to each other, featuring a hydrophobic domain in the middle. The cyanobacterial consensus sequence exhibits significant homology to the presumed product of ORF32 in the chloroplast DNA of liverwort (Marchantia polymorpha), but no homologous ORF is present in the chloroplast DNA of tobacco or rice. Since this protein appears to interact strongly with the PS I reaction center complex, it may play some role in the function and maintenance of the structure of PS I.