Nucleotide sequence of the heme subunit of flavocytochrome c from the purple phototrophic bacterium, Chromatium vinosum. A 2.6-kilobase pair DNA fragment contains two multiheme cytochromes, a flavoprotein, and a homolog of human ankyrin.

The gene for the cytochrome subunit of Chromatium vinosum flavocytochrome c (sulfide dehydrogenase) was cloned from an EcoRI digest of chromosomal DNA. The mature cytochrome subunit contains 175 amino acid residues and two heme binding sites in agreement with the previously reported amino acid sequence. There is also a signal peptide of 25 residues, which apparently directs the protein to the periplasmic space. There are two open reading frames upstream of the heme subunit gene, which encode a tetraheme cytochrome c and a homolog of human ankyrin. The gene for the flavoprotein subunit of flavocytochrome c is in frame 15 nucleotides downstream of the stop codon for the cytochrome gene. Messenger RNA was isolated from malate grown cells. The transcript is approximately 3 kilobases in size and does not hybridize with a probe containing the tetraheme cytochrome gene and part of the ankyrin homolog gene. The heme subunit and flavoprotein subunit genes thus appear to form an operon. The flavoprotein subunit has a 30-residue signal peptide. The clone ends 95 amino acids into the N-terminal sequence of the mature flavoprotein subunit (which should contain about 400 residues). The apparently periplasmic location of flavocytochrome c has important consequences for the presumed function as a sulfide dehydrogenase, because sulfur, which is the product of oxidation, is stored in the cytoplasm. Our results on the location of the enzyme are incompatible with this function.