A basic cytochrome was isolated from the phytomastigophorean protozoan Euglena gracilis and a similar protein from the zoomastigophorean protozoan Crithidia oncopelti. In both cases chromatography on CM-cellulose in first the reduced and then the oxidized form proved to be an efficient means of purification. The two cytochromes can be classed in the cytochrome c family but they have certain atypical features. The alpha peak of the absorption spectrum is shifted towards the red and is asymmetrical. The pyridine ferrohaemochrome has an alpha-peak maximum intermediate between that of c-type cytochromes and proteins containing protohaem IX. The test for free vinyl groups was positive. The amino acid sequences of the two cytochromes were determined. Attention is drawn in the text to those parts of the evidence that are less satisfactory. Both sequences are homologous with the family of cytochrome c, but are unusual in having only one cysteine residue so that the haem is attached through only one thioether bond. Detailed evidence for the amino acid dequences of the two proteins has been deposited as Supplementary Publication SUP 50042 (70 pages) at the British Library (Lending Division) (formerly the National Lending Library for Science and Technology), Boston Spa, Wetherby, Yorks. LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1975) 145, 5.

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1. The histones of Euglena gracilis were separated by gel filtration into five fractions. 2. Each fraction was characterized in terms of its electrophoretic, solubility and compositional properties. 3. Euglena gracilis clearly contains histones corresponding to vertebrate H1, H2B, H3 and H4 fractions, although they all differ in containing more lysine. 4. The remaining Euglena histone is considered to be homologous to vertebrate histone H2A, but it differs in having a much higher ratio of lysine to arginine. 5. The Euglena histone H1 appears to be lacking in aspartic acid. 6. Electrophoresis in the presence of sodium dodecyl sulphate indicates that the molecular weights of the Euglena histones are close to those of the homologous vertebrate histones.

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