The amino acid sequence of cytochrome c ‐556 from Agrobacterium tumefaciens strain Apple 185

The evidence for the amino acid sequence of cytochrome c ‐556 from Agrobacterium tumefaciens strain Apple 185 is reported. The sequence was determined by manual Edman degradation of tryptic and chymotryptic peptides using the DABITC/PITC double‐coupling method; some peptides were further cleaved by partial acid hydrolysis and with Staphylococcus aureus protease. The sequence overlaps 13–15, 83–85 and 106–108 as well as the region 113–118 involving the haem‐binding sequence Cys‐Xaa‐Xaa‐Cys‐His were deduced by homology with cytochrome c ‐556 from Agrobacterium tumefaciens strain B2a. The identity of histidine at position 6 has been inferred from fast‐atom‐bombardment experiments on the N ‐terminal tryptic peptide, and Asp‐63 was deduced from the electrophoretic mobility of the peptides in which it occurs. The cytochrome from A. tumefaciens Apple 185 contains 125 amino acids of which 71 are identical in the protein from strain B2a. Together with cytochrome c ‐556 from the photosynthetic prokaryote Rhodopseudomonas palustris strain 2.1.37, the presently studied protein is the third known example of a monohaem class II cytochrome of the low‐spin type having the single haem group covalently linked near the C terminus of the polypeptide chain. The only methionine residue in the Apple protein, methionine‐13, is the most likely candidate to be the sixth haem ligand and therefore to be responsible for the low‐spin character of the haem iron.