Primary structure characterization of a Rhodocyclus tenuis diheme cytochrome c reveals the existence of two different classes of low-potential dihemecytochromes c in purple phototropic bacteria

The complete amino acid sequence of a 26-kDa low redox potential cytochrome c-551 from Rhodocyclus tenuis was determined by a combination of Edman deg radation and mass spectrometry, There are 240 residues including two heme b inding sites at positions 41, 44, 128, and 132, There is no evidence for ge ne doubling. The only known homolog of Re. tenuis cytochrome c-551 is the d iheme cytochrome c-552 from Pseudomonas stutzeri which contains 268 residue s and heme binding sites at nearly identical positions. There is 44% overal l identity between the Rc. tenuis and Ps. stutzeri cytochromes with 10 inte rnal insertions and deletions. The Ps. stutzeri cytochrome is part of a den itrification gene cluster, whereas Rc. tenuis is incapable of denitrificati on, suggesting different functional roles for the cytochromes, Histidines a t positions 45 and 133 are the fifth heme ligands and conserved histidines at positions 29, 209, and 218 and conserved methionines at positions 114 an d 139 are potential sixth heme ligands, There is no obvious homology to the low-potential diheme cytochromes characterizes from other purple bacterial species such as Rhodobacter sphaeroides, There are therefore at least two classes of low-potential diheme cytochromes c found in phototrophic bacteri a. There is no more than 11% helical secondary structure in Rc. tenuis cyto chrome c-551 suggesting that there is no relationship to class I or class I I c-type cytochromes, (C) 2000 Academic Press.