Bacterial cytochromes c biogenesis.

Abstract

We report the primary sequence analyses of two loci, hel and ccl, whose gene products are required specifically for the biogenesis of c-type cytochromes in the Gram-negative photosynthetic bacterium Rhodobacter capsulatus. Genetic and molecular analyses show that the hel locus contains at least four genes, helA, helB, helC, and orf52, and the ccl locus contains two genes, cell and ccl2, that are essential for cytochromes c biogenesis. HelA is homologous to a class of proteins called ABC transporters and helA, belB, and helC are proposed to encode an export complex. Cytochrome C2-alkaline phosphatase gene fusions were used to show that apocytochrome C2 synthesis and secretion are not affected by the hel and ccl defects. Cell and Ccl2 possess typical signal sequences to direct them to the periplasm. The periplasmic orientation of Cell was confirmed using a CcU-alkaline phosphatase gene fusion. The CcU-alkaline phosphatase gene fusion analysis also demonstrated that Cell does not require hel genes for its synthesis and secretion. Cell is homologous to proteins encoded by chloroplast and mitochondrial genes, suggesting analogous functions in these organelles. Taken together, these results support the hypothesis that the hel-encoded proteins are required for the export of heme to the periplasm where it is subsequently ligated to the c-type apocytochromes.

Translocation to the Periplasm and Signal Sequence Cleavage of Preapocytochrome c Depend on Sec and Lep, but not on the ccm gene products

“…However, this transporter may be required to export another molecule, which may be either heme, as suggested previously (Ramseier et al, 1991;Beckman et al, 1992;Thony-Meyer et al, 1994), or an additional, unknown factor assisting heme ligation in the periplasm.…”

“…There is good evidence that apocytochrome c is first translocated to the periplasm before it is converted to holocytochrome (Page and Ferguson, 1990;Beckman et al, 1992;Sambongi and Ferguson, 1994;ThonyMeyer et al, 1996). Eight genes called ccmABCDEFGH (for cytochrome c maturation) are specifically required for the formation of holocytochrome c in Escherichia coli Grove et al, 1996b).…”

Involvement of products of the nrfEFG genes in the covalent attachment of haem c to a novel cysteine–lysine motif in the cytochrome c552 nitrite reductase from Escherichia coli

“…These are the final three genes of the nrfABC-DEFG operon (encoding proteins of the energy-conserving pathway for nitrite reduction to ammonia in which formate is the electron donor; Hussain et al, 1994;Grove et al, 1996a,b) and the eight ccm genes located downstream of the nap operon at minute 47 on the E. coli chromosome (Thö ny-Meyer et al, Grove et al, 1996a). NrfE and NrfF are similar to Ccl1 and Ccl2 of Rhodobacter capsulatus, and also to CycK and CycL from Bradyrhizobium japonicum, Rhizobium leguminosarum, Rhizobium etli, Rhizobium meliloti and Pseudomonas fluorescens, all of which are essential for the synthesis of c-type cytochromes in these bacteria and have been suggested to be components of a periplasmic haem lyase complex (Ramseier et al, 1991;Beckman et al, 1992;Thö ny-Meyer et al, 1994Delgado et al, 1995;Ritz et al, 1995;Kereszt et al, 1995;Gaballa et al, 1996;Tabche et al, 1996;Yang et al, 1996). NrfE, like Ccl1, is predicted to be a membrane-spanning protein; NrfF is predicted to be a hydrophilic, periplasmic protein.…”

Mechanism of substrate specificity in Bacillus subtilis ResA, a thioredoxin-like protein involved in cytochrome c maturation

“…The use of thioredoxinderived reducing equivalents is a fundamental requirement for a variety of bacterial extracellular and periplasmic activities, such as the activities of the disulfide bond forming (Dsb) system, the crosslinking of spore coat proteins, and CCM. These processes cannot be controlled by concentration dependence alone; and, in the case of CCM, genetic elimination of the terminal thioredoxin-like protein has been shown to specifically abolish maturation of c-type cytochromes for a number of systems (5,14,31,32). Crow et al (21) presented a structure-based theory that ResA (system II CCM) uses redox-coupled conformational changes to select its substrate.…”