Cytochromes c biogenesis in a photosynthetic bacterium requires a periplasmic thioredoxin-like protein.
Rhodobacter capsulatus is a Gram-negative photosynthetic bacterium that requires c-type cytochromes for photosynthetic electron transport. Our studies demonstrate that the gene helX is required for the biogenesis of c-type cytochromes in R. capsulatus. A helX chromosomal deletion mutant cannot grow photosynthetically, due to a deficiency of all c-type cytochromes. The predicted amino acid sequence of the helX gene product (176 residues) is related to that of thioredoxin and shares active-site homology with protein disulfide isomerase. Cytochrome c2-alkaline phosphatase gene fusions are used to show that HelX is not required for the transcription, translation, or secretion of apocytochrome c2. HelX-alkaline phosphatase and HelX-beta-galactosidase gene fusions are used to demonstrate that HelX is a periplasmic protein, which is consistent with the presence of a typical signal sequence in HelX. Based on these results, we propose HelX functions as a periplasmic disulfide oxidoreductase that is essential for cytochromes c biogenesis. This role is in accordance with the observation that both heme and the cysteines of apocytochromes c (Cys-Xaa-Yaa-Cys-His) must be in the reduced state for covalent linkage between the two moieties to occur.
Department of Biology, Washington University, St. Louis, MO 63130.
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Kranz
D000469:Alkaline Phosphatase / Q000235:genetics / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002872:Chromosome Deletion; D002876:Chromosomes, Bacterial; D003001:Cloning, Molecular; D003574:Cytochrome c Group / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D004269:DNA, Bacterial / Q000235:genetics*; D004926:Escherichia coli / Q000235:genetics; D005798:Genes, Bacterial; D007700:Kinetics; D008969:Molecular Sequence Data; D010788:Photosynthesis; D010957:Plasmids; D011487:Protein Conformation; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000378:metabolism; D015183:Restriction Mapping; D016354:Rhodobacter capsulatus / Q000235:genetics* / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013053:Spectrophotometry; D013879:Thioredoxins / Q000235:genetics / Q000378:metabolism*; D001616:beta-Galactosidase / Q000235:genetics / Q000378:metabolism
D003574:Cytochrome c Group; D004269:DNA, Bacterial; D011993:Recombinant Fusion Proteins; D013879:Thioredoxins; D000469:Alkaline Phosphatase; D001616:beta-Galactosidase