The amino acid sequences of cytochrome b of complex III from five different mitochondrial sources (human, bovine, mouse, yeast, and Aspergillus nidulans) and the chloroplast cytochrome b6 from spinach show a high degree of homology. Calculation of the distribution of hydrophobic residues with a "hydropathy" function that is conserved in this family of proteins implies that the membrane-folding pattern of the 42-kilodalton (kDa) mitochondrial cytochromes involves 8-9 membrane-spanning domains. The smaller 23-kDa chloroplast cytochrome appears to fold in five spanning domains that are similar to the first five of the mitochondria. Four highly conserved histidines are considered to be the likely ligands for the two hemes. The positions of the histidines along the spanning segments and in a cross section of the membrane-spanning alpha helices implies that two ligand pairs, His-82-His-197/198 and His-96-His-183, bridge the spanning peptides II and V, and the two hemes reside on opposite sides of the hydrophobic membrane core. In addition, the 17-kDa protein of the chloroplast b6-f complex appears to contain one or more of the functions of the COOH-terminal end of the mitochondrial cytochrome b polypeptide.