| A mutant of the cyanobacterium Synechocystis sp. PCC 6803, unable to grow photoautotrophically, was generated through the use of recombinant DNA technology. This mutant, named M8, was able to grow photobeterotrophically in the presence of glucose. Mass spectrometric analysis revealed that the mutant was capable of transporting inorganic carbon and generating an intracellular Ci pool; however, it was unable to utilize the pool for carbon fixation, as evidenced by the absence of O2 evolution. The mutant was unable to donate electrons to the artificial PSII electron acceptor benzoquinone, suggesting a defect in the photosynthetic linear electron transport chain. The failure of hydroxylamine, which donates electrons to PSII, to relieve the block suggests that the defect is in PSII itself. Fluorescence emission spectroscopy at 77K revealed the absence of a peak at 695 nm in mutant cells upon excitation with phycobilin or chlorophyll-absorbed light, providing further evidence for a defect within the PSII core complex. The defect prevented the transfer of excitation energy from the phycobilisome terminal emitter to the PSII core and consequently caused an impairment in state transitions. |
