The photosynthetic membrane proteome of Rhodobacter sphaeroides R-26.1 exposed to cobalt

Cells of the carotenoidless strain R-26.1 of Rhodobacter sphaeroides were grown in the presence of a high concentration (5 mM) of cobalt ions. The photosynthetic intracytoplasmic membranes were isolated and investigated by proteomic analysis using non-denaturating blue native electrophoresis in combination with LC-ESI-MS/MS. Comparison with intracytoplasmic membranes of cells grown under control conditions showed a change in the relative amount of proteins belonging to the photosynthetic apparatus, with net downregulation of light-harvesting complexes and increased concentration of the nude reaction center (RC), as well as upregulation of enzymes related to chemoorganotrophy. These effects represent possible bacterial adaptation so as to retrieve energy for metabolic processes from sources alternative to less efficient photosynthesis. The influence of cobalt on the photochemistry of the RC in cell extracts was also investigated by charge recombination. The kinetics of the charge recombination reaction was found to be slower in extracts from cells exposed to Co2+, indicating that the reorganization of the photosynthetic apparatus also involves its photochemical core.