Absence of Thiol-Disulfide Oxidoreductase DsbA Impairs cbb 3 -Type Cytochrome c Oxidase Biogenesis in Rhodobacter capsulatus
The thiol-disulfide oxidoreductase DsbA carries out oxidative folding of extra-cytoplasmic proteins by catalyzing the formation of intramolecular disulfide bonds. It has an important role in various cellular functions, including cell division. The purple non-sulfur bacterium Rhodobacter capsulatus mutants lacking DsbA show severe temperature-sensitive and medium-dependent respiratory growth defects. In the presence of oxygen, at normal growth temperature (35°C), DsbA - mutants form colonies on minimal medium, but they do not grow on enriched medium where cells elongate and lyse. At lower temperatures (i.e., 25°C), cells lacking DsbA grow normally in both minimum and enriched media, however, they do not produce the cbb 3 -type cytochrome c oxidase ( cbb 3 -Cox) on enriched medium. Availability of chemical oxidants (e.g., Cu 2+ or a mixture of cysteine and cystine) in the medium becomes critical for growth and cbb 3 -Cox production in the absence of DsbA. Indeed, addition of Cu 2+ to the enriched medium suppresses, and conversely, omission of Cu 2+ from the minimal medium induces, growth and cbb 3 -Cox defects. Alleviation of these defects by addition of redox-active chemicals indicates that absence of DsbA perturbs cellular redox homeostasis required for the production of an active cbb 3 -Cox, especially in enriched medium where bioavailable Cu 2+ is scarce. This is the first report describing that DsbA activity is required for full respiratory capability of R. capsulatus , and in particular, for proper biogenesis of its cbb 3 -Cox. We propose that absence of DsbA, besides impairing the maturation of the c-type cytochrome subunits, also affects the incorporation of Cu into the catalytic subunit of cbb 3 -Cox. Defective high affinity Cu acquisition pathway, which includes the MFS-type Cu importer CcoA, and lower production of the c-type cytochrome subunits lead together to improper assembly and degradation of cbb 3 -Cox.