Cytochrome c biogenesis in Bacillus subtilis. Identification and analysis of ccdA

Abstract: Cytochromes function in electron transfer reactions, e.g. in respiratory systems, and contain haem as a prosthetic group. Cytochromes of the c-type differ from other cytochromes in that haem is covalently attached, via thioether linkages, to two cysteine residues in a conserved sequence motif in the protein, Cys-Xaa-Yaa-Cys-His. The functional location of c-type cytochromes in bacteria is on the outer side of the cytoplasmic membrane. Haem and apocytochrome c polypeptide are synthesised in the cytoplasm but assembly of cytochrome c occurs on the outer side of the cytoplasmic membrane. Little is known about biogenesis of cytochromes c in Gram-positive bacteria. Bacillus subtilis is a Gram-positive, endospore forming, bacterium. It contains four membrane-bound c-type cytochromes. A mutant with a pleiotropic cytochrome c deficiency has been isolated and characterised, leading to the identification of the ccdA gene. The CcdA protein is required for a late step of both cytochrome c and spore synthesis. The ccdA gene is co-transcribed with two downstream genes, yneI and yneJ. These genes, which are also transcribed from their own promoters, are not required for cytochrome c or spore synthesis. The ccdA promoter is weak and increases in activity at the end of the exponential growth phase. CcdA is a 26 kDa polytopic integral membrane protein with sequence similarity to the central, hydrophobic, domain of the larger E. coli DsbD protein. DsbD functions to reduce a disulphide isomerase(s) in the periplasm. CcdA homologues are found in Bacteria, Archae and plants. Conserved cysteine residues in these proteins (and also in DsbD-like proteins) may be involved in transfer of reducing equivalents across membranes. Redox chemistry on the outer side of the B. subtilis cytoplasmic membrane might be important for both cytochrome c and spore-synthesis, and affected in a ccdA null mutant.