Cytochrome c-552 from Euglena gracilis was purified and the amino acid sequence determined. The protein is a single peptide chain of 87 residues with the haem prosthetic group bound through two thioether linkages to two cysteine residues near the amino-terminal region. The amino acid sequence shows some similarities to mitochondrial cytochrome c and to two prokaryote c-type cytochromes. The sequence, taken with the known characteristics of cytochrome c-552, indicates that it is an f-type cytochrome. The possible functional and evolutionary significance of these features in common is discussed. Detailed evidence for the amino acid sequence of Euglena cytochrome f has been deposited as Supplementary Publication SUP 50027 at the British Library, Lending Division (formerly the National Lending Library for Science and Technology), Boston Spa, Yorks. LS23 7QB, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1973) 131, 5.

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