Cyanobacteria are ancient photosynthetic prokaryotes that have adapted successfully to adverse environments including high-light irradiation. Although it is known that the repair of photodamaged photosystem II (PSII) in the organisms is a highly regulated process, our knowledge of the molecular components that regulate each step of the process is limited. We have previously identified a hypothetical protein Slr0151 in the membrane fractions of cyanobacterium Synechocystis sp. PCC 6803. Here, we report that Slr0151 is involved in PSII repair of the organism. We generated a mutant strain (Δslr0151) lacking the protein Slr0151 and analyzed its characteristics under normal and high-light conditions. Targeted deletion of slr0151 resulted in decreased PSII activity in Synechocystis. Moreover, the mutant exhibited increased photoinhibition due to impairment of PSII repair under high-light condition. Further analysis using in vivo radioactive labeling and 2-D blue native/sodium dodecylsulfate polyacrylamide gel electrophoresis indicated that the PSII repair cycle was hindered at the levels of D1 synthesis and disassembly and/or assembly of PSII in the mutant. Protein interaction assays demonstrated that Slr0151 interacts with D1 and CP43 proteins. Taken together, these results indicate that Slr0151 plays an important role in regulating PSII repair in the organism under high-light stress condition.

The unicellular photosynthetic model-organism cyanobacterium Synechocystis sp. PCC6803 can grow photoautotrophically using CO 2 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. Molecular & Cellular