Molecular cloning and nucleotide sequence of a cDNA encoding Euglena gracilis Cytochrome C₁

The amino acid sequence of the mature protein of Euglena gracilis cytochrome c₁ was determined by sequencing of its cDNA. A cDNA expression library was constructed from Euglena poly(A)⁺ RNA in phage λgt11 and screened with an antiserum raised against cytochrome c₁ polypeptide isolated from purified E. gracilis complex III. An isolated cDNA clone consisted of 872 base pairs and encoded the mature protein with 243 amino acids. The deduced amino acid sequence contained the unusual heme binding sequence -Phe-Ala-Pro-Cys-His- (Mukai, K. et al (1989) Eur. J. Biochem. 178, 649-656) instead of the typical sequence, -Cys-X-Y-Cys-His-, commonly found in C-type cytochromes. Comparison of the sequence with those of several other cytochromes c₁ revealed that Euglena cytochrome c₁ conserved the residues probably ligating heme-iron, those supposed to interact with cytochrome c and regions anchoring the mitochondrial inner membrane.