Purification and primary structure of cytochrome f from the cyanobacterium, Plectonema boryanum

The amino acid sequence of the soluble c-type cytochrome, cytochrome f, from the cyanobacterium Plectonema boryanum (also called Phormidium luridum or Schizothrix calcicola) has been determined. The proposed sequence consists of one polypeptide chain of 85 residues and has three Asn-Gly linkages. Partly due to the presence of these Asn-Gly bonds, which readily undergo rearrangement, proteolytic digestion on the small amount of protein available was unsatisfactory. The structure was determined partly by a combination of chemical cleavage and automatic sequencing techniques. A new technique for conserving material by cyanogen bromide cleavage of residual polypeptide after automatic degradation is described. The possible evolutionary significance of primary structure comparisons with other cytochromes f is discussed.