Cytochrome c-550 of Thiobacillus versutus functions as an electron transfer protein in a chain of redox proteins that enables I: versutus to grow on methylamine. It is a single-heme protein of 134 residues, related to mitochondrial cytochrome c. Cytochrome c-550, as well as several other bacterial c2-type cytochromes, contain a C-terminal extension of 13-16 amino acids of unknown function, compared to mitochondrial cytochrome c.NMR experiments were performed to obtain structural and dynamic information on the protein in solution. For this purpose, I: versutus cytochrome c-550 was labeled with '*N and I3C using I3C-methanol grown Paracoccus denitrificans as a host for heterologous expression. NMR assignments were obtained for the 'H, "N, and I3C nuclei in the backbone and the P-positions of the protein and the secondary structure was determined. "N-relaxation studies were performed to characterize the dynamic properties of the protein.The results indicate that the main part of I: versutus ferrocytochrome c-550 exists in solution as a rigid, well-ordered molecule with a secondary structure that is very similar to that of P. denitrificans cytochrome c-550, as observed in crystals. The C-terminal extension, however, is unstructured and highly mobile. The possible origin and function of the extension are discussed.