Cloning and expression in Escherichia coli of the cytochrome c552 gene from Thermus thermophilus HB8 -: Evidence for genetic linkage to an ATP-binding cassette protein and initial characterization of the cycA gene products

We report sequence of Thermus thermophilus HB8 DNA containing the gene (cycA) for cytochrome c(552) and a gene (cycB) encoding a protein homologous with one subunit of an ATP-binding cassette transporter. The cycA gene encodes a 17-residue N-terminal signal peptide with following amino acid sequence identical to that reported by (Titani, K., Ericsson, L. H., Hon-nami, K., and Miyazawa, T. (1985) Biochem. Biophys, Res. Commun. 128, 781-787), A modified cycA was placed under control of the T7 promoter and expressed in Escherichia coli, Protein identical to that predicted from the gene sequence was found in two heme C-containing fractions. Fraction rC(552), characterized by an alpha-band at 552 nm, contains similar to 60-70% of a protein highly similar to native cytochrome c(552) and similar to 30-40% of a protein that contains a modified heme, Cytochrome rC(552) is monomeric and is an excellent substrate for cytochrome ba(3). Cytochrome rC(557) is characterized by an alpha-band at 557 nm, contains similar to 90% heme C and similar to 10% of non-C heme, exists primarily as a homodimer, and is essentially inactive as a substrate for cytochrome ba(3), We suggest that rC(557) is a "conformational isomer" of rC(552) having non-native, axial ligands to the heme iron and an "incorrect" protein fold that is stabilized by homodimer formation.