Evidence for the amino-acid sequence of Crithidia fasciculata Cytochrome c555

Evidence is presented which indicates that the amino acid sequence of cytochrome c555 from Crithidia fasciculata differs at sixteen positions from that of cytochrome c557 from Crithidia oncopelti. 101 residues were identified by dansyl-Edman degradation, carboxypeptidase digestion or considerations of the specificity of trypsin and of these, thirteen were found to be different from C. oncopelti cytochrome c557. The remaining 11 residues found in the amino acid composition of the trypic peptides were aligned on the basis of homology with cytochrome c557 and three further differences are proposed. The total of sixteen amino acid differences is surprising in view of the morphological and biochemical similarities of these organisms, and illustrates the problem of taxonomy of morphologically simple organisms. In both cytochromes only one cysteine residue is involved in the attachment of the protein to the prosthetic group.