Purification and Primary Structure of Cytochrome c‐552 from the Cyanobacterium, Synechococcus PCC 6312

Cytochrome c-552 (soluble 'cytochrome f') from the unicellular cyanobacterium Synechococcus PCC 6312 (ATCC 27167) was purified and the primary structure determined. The proposed sequence consists of one polypeptide chain of 87 residues. The sequence was determined by a combination of chemical and enzymatic cleavage, manual and automatic sequencing and mass spectroscopy. This is the first amino acid sequence of this cytochrome from a unicellular cyanobacterium to be determined in a study of the variation in primary structure between phylogenetically distant cyanobacteria. The sequence is compared to the primary structures of the cytochrome from filamentous cyanobacteria and from eukaryotic algae. The significance of these sequence comparisons to the current hypotheses concerning the origin of eukaryotic cells and their chloroplasts is discussed.