A Δzwf (glucose-6-phosphate dehydrogenase) mutant of the cyanobacteriumSynechocystis sp. PCC 6803 exhibits unimpaired dark viability

Metabolic flux of the oxidative pentose phosphate pathway under low light conditions in Synechocystis sp. PCC 6803

The role of the oxidative pentose phosphate pathway (oxPPP) in Synechocystis sp. PCC 6803 under mixotrophic conditions was investigated by ¹³C metabolic flux analysis. Cells were cultured under low (10 μmol m^-2 s^-1) and high light intensities (100 μmol m^-2 s^-1) in the presence of glucose. The flux of CO₂ fixation by ribulose bisphosphate carboxylase/oxygenase under the high light condition was approximately 3-fold higher than that under the low light condition. Although no flux of the oxPPP was observed under the high light condition, flux of 0.08-0.19 mmol gDCW^-1 h^-1 in the oxPPP was observed under the low light condition. The balance between the consumption and production of NADPH suggested that approximately 10% of the total NADPH production was generated by the oxPPP under the low light condition. The growth phenotype of a mutant with deleted zwf, which encodes glucose-6-phosphate dehydrogenase in the oxPPP, was compared to that of the parental strain under low and high light conditions. Growth of the Δzwf mutant nearly stopped during the late growth phase under the low light condition, whereas the growth rates of the two strains were identical under the high light condition. These results indicate that NADPH production in the oxPPP is essential for anabolism under low light conditions. The oxPPP appears to play an important role in producing NADPH from glucose and ATP to compensate for NADPH shortage under low light conditions.

Sll1783, a monooxygenase associated with polysaccharide processing in the unicellular cyanobacterium Synechocystis PCC 6803

Cyanobacteria play a pivotal role as the primary producer in many aquatic ecosystems. The knowledge on the interacting processes of cyanobacteria with its environment - abiotic and biotic factors - is still very limited. Many potential exocytoplasmic proteins in the model unicellular cyanobacterium Synechocystis PCC 6803 have unknown functions and their study is essential to improve our understanding of this photosynthetic organism and its potential for biotechnology use. Here we characterize a deletion mutant of Synechocystis PCC 6803, Δsll1783, a strain that showed a remarkably high light resistance which is related with its lower thylakoid membrane formation. Our results suggests Sll1783 to be involved in a mechanism of polysaccharide degradation and uptake and we hypothesize it might function as a sensor for cell density in cyanobacterial cultures.

Functional proteomic discovery of Slr0110 as a central regulator of carbohydrate metabolism in Synechocystis species PCC6803

The unicellular photosynthetic model-organism cyanobacterium Synechocystis sp. PCC6803 can grow photoautotrophically using CO2 or heterotrophically using glucose as the sole carbon source. Several pathways are involved in carbon metabolism in Synechocystis, and the concerted regulation of these pathways by numerous known and unknown genes is critical for the survival and growth of the organism. Here, we report that a hypothetical protein encoded by the open reading frame slr0110 is necessary for heterotrophic growth of Synechocystis. The slr0110-deletion mutant is defective in glucose uptake, heterotrophic growth, and dark viability without detectable defects in autotrophic growth, whereas the level of photosystem II and the rate of oxygen evolution are increased in the mutant. Quantitative proteomic analysis revealed that several proteins in glycolysis and the oxidative pentose phosphate pathway are down-regulated, whereas proteins in photosystem II and phycobilisome are significantly up-regulated, in the mutant. Among the down-regulated proteins are glucose transporter, glucokinase, glucose-6-phosphate isomerase, and glucose-6-phosphate dehydrogenase and its assembly protein OpcA, suggesting that glycolysis, oxidative pentose phosphate, and glycogen synthesis pathways are significantly inhibited in the mutant, which was further confirmed by enzymatic assays and quantification of glycogen content. These findings establish Slr0110 as a novel central regulator of carbon metabolism in Synechocystis, and shed light on an intricate mechanism whereby photosynthesis and carbon metabolism are well concerted to survive the crisis when one or more pathways of the system are impaired.