Investigating oxygen evolution in the absence of photosystem I in the cyanobacterium Synechocystis 6803

Abstract

The photosynthetic electron transport pathway from H2O to NADP+ in higher plants, algae, and cyanobacteria requires the PSII and PSI reaction center complexes, the cytochrome bf complex, the mobile electron carriers plastoquinone, plastocyanin and ferredoxin, and the peripheral ferredoxin-NADP+ oxidoreductase. Surprisingly, we have found that photosystem I deletion mutant strains of the cyanobacterium Synechocystis 6803 (DPSI) are able to evolve O2 for over one hour at rates comparable to the O2 evolution rate of wild type cells under physiological conditions. However, the DPSI are unable to grow photoautotrophically. We have shown that the unknown PSII O2 evolution pathway in the DPSI mutant cells is sensitive to the PSII inhibitor DCMU, but insensitive to the cytochrome bf inhibitor DBMIB. In addition, we have shown that the unknown PSII O2 evolution pathway in the DPSI mutants is highly cyanide sensitive. The DPSI mutants show a net oxygen evolution in the light, so O2 cannot be the terminal electron acceptor. The DPSI mutants do not release H2, which eliminates hydrogen ions as the final electron acceptor. Experiments using C14 labeled bicarbonate indicate that light induced CO2 uptake accounts for about 20% of the electrons originating from water oxidation. The addition of glucose to glucose starved cells restores oxygen evolution, which raises the possibility that a glucose metabolic intermediate may serve as the terminal electron acceptor of the unknown PSII O2 evolution pathway