from Rhodobacter sphaeroides

Cytochrome c0 isolated from Rhodobacter sphaeroides strain R26 (RSCP) crystallizes as a dimer of two identical 14-kDa subunits, in trigonal space group P31, with cell parameters a; b D 48:10 ˚A, c D 115:80 ˚A. The crystal structure of RSCP has been solved by molecular replacement using cytochrome c0 from Rhodobacter capsulatus (PDB ID: 1CPQ) as a search model. To ensure effective phase bias removal, the RSCP model was iteratively built into maps generated by a modified wARP procedure, Shake&wARP. The 1.8 ˚A model (PDB ID: 1GQA) has been refined to an R D 0:204 and freeR D 0:254. Each subunit consists of four antiparallel fi-helices, with the pentacoordinate heme covalently bound to a C---X---Y---C---H motif near the C-terminus. F14, located on helix A, blocks direct access to what would be the sixth “distal ” ligand binding site of the heme. The dimer subunits form a flattened “X ” shape, intermediate between the Type 1 and Type 2 cytochromes c0. The presence of the aromatic F14 and a deep channel between helices B and C places RSCP into Group 1 cytochromes c0. Clear electron density has revealed that the amino acid sequences for the cytochrome c0 from strains R26 and 2.4.1 are identical. KEY WORDS: Cytochrome c0; heme protein; four helix bundle; Rhodobacter sphaeroides