The dithiol:disulfide oxidoreductases DsbA and DsbB of Rhodobacter capsulatus are not directly involved in cytochrome c biogenesis, but their inactivation restores the cytochrome c

• Published in: Journal of bacteriology Vol. 185; no. 11; p. 3361
• Main Authors: Deshmukh, Meenal, Turkarslan, Serdar, Astor, Donniel, Valkova-Valchanova, Maria, Daldal, Fevzi
• Format: Journal Article
• Language: English
• Published: Washington American Society for Microbiology 01.06.2003
• Subjects: Bacteria; Bacteriology; Mutation; Proteins
• ISSN: 0021-9193; 1098-5530

The cytoplasmic membrane protein CcdA and its homologues in other species, such as DsbD of Escherichia coli, are thought to supply the reducing equivalents required for the biogenesis of c-type cytochromes that occurs in the periplasm of gram-negative bacteria. CcdA-null mutants of the facultative phototroph Rhodobacter capsulatus are unable to grow under photosynthetic conditions (Ps-) and do not produce any active cytochrome c oxidase (Nadi-) due to a pleiotropic cytochrome c deficiency. However, under photosynthetic or respiratory growth conditions, these mutants revert frequently to yield Ps+ Nadi+ colonies that produce c-type cytochromes despite the absence of CcdA. Complementation of a CcdA-null mutant for the Ps+ growth phenotype was attempted by using a genomic library constructed with chromosomal DNA from a revertant. No complementation was observed, but plasmids that rescued a CcdA-null mutant for photosynthetic growth by homologous recombination were recovered. Analysis of one such plasmid revealed that the rescue ability was mediated by open reading frame 3149, encoding the dithiol:disulfide oxidoreductase DsbA. DNA sequence data revealed that the dsbA allele on the rescuing plasmid contained a frameshift mutation expected to produce a truncated, nonfunctional DsbA. Indeed, a dsbA ccdA double mutant was shown to be Ps+ Nadi+, establishing that in R. capsulatus the inactivation of dsbA suppresses the c-type cytochrome deficiency due to the absence of ccdA. Next, the ability of the wild-type dsbA allele to suppress the Ps+ growth phenotype of the dsbA ccdA double mutant was exploited to isolate dsbA-independent ccdA revertants. Sequence analysis revealed that these revertants carried mutations in dsbB and that their Ps+ phenotypes could be suppressed by the wild-type allele of dsbB. As with dsbA, a dsbB ccdA double mutant was also Ps+ Nadi+ and produced c-type cytochromes. Therefore, the absence of either DsbA or DsbB restores c-type cytochrome biogenesis in the absence of CcdA. Finally, it was also found that the DsbA-null and DsbB-null single mutants of R. capsulatus are Ps+ and produce c-type cytochromes, unlike their E. coli counterparts, but are impaired for growth under respiratory conditions. This finding demonstrates that in R. capsulatus the dithiol:disulfide oxidoreductases DsbA and DsbB are not essential for cytochrome c biogenesis even though they are important for respiration under certain conditions. [PUBLICATION ABSTRACT]