CHARACTERIZATION AND AMINO-ACID-SEQUENCE OF CYTOCHROME C-550 FROM THIOSPHAERA-PANTOTROPHA

A cytochrome c-550, with mid-point potential +265 mV, has been purifie d from Thiosphaera pantotropha. The cytochrome was recognised by antib odies to Paracoccus denitrificans cytochrome c-550, but the two protei ns were not immunologically identical. Amino acid sequencing of the cy tochrome c-550 showed 85.9% and 95.5% identities, respectively, with t he cytochromes c-550 of P. denitrificans and Thiobacillus versutus; th ese are amongst the highest values reported for similarities between c lass I c-type cytochromes of the c, group. These similarities are cons istent with the published values of 85% for the overall DNA similarity of P. denitrificans and T. pantotropha, but contrast with published 1 6S rRNA analyses which indicate identity between I: pantotropha and P. denitrificans and 97.5% similarity of T. versutus with these two orga nisms. Analysis by plasma-desorption mass spectrometry of the peptide containing the haem-binding motif isolated from the apocytochrome has shown that an Hg atom binds to one or both of the two thiol groups.