Abstract
Light drives the production of chemical energy and reducing equivalents in photosynthetic organisms required for the assimilation of essential nutrients. This process also generates strong oxidants and reductants that can be damaging to the cellular processes, especially during absorption of excess excitation energy. Cyanobacteria, like other oxygenic photosynthetic organisms, respond to increases in the excitation energy, such as during exposure of cells to high light (HL) by the reduction of antenna size and photosystem content. However, the mechanism of how Synechocystis sp. PCC 6803, a cyanobacterium, maintains redox homeostasis and coordinates various metabolic processes under HL stress remains poorly understood. In this study, we have utilized time series transcriptome data to elucidate the global responses of Synechocystis to HL. Identification of differentially regulated genes involved in the regulation, protection, and maintenance of redox homeostasis has offered important insights into the optimized response of Synechocystis to HL. Our results indicate a comprehensive integrated homeostatic interaction between energy production (photosynthesis) and energy consumption (assimilation of carbon and nitrogen). In addition, measurements of physiological parameters under different growth conditions showed that integration between the two processes is not a consequence of limitations in the external carbon and nitrogen levels available to the cells. We have also discovered the existence of a novel glycosylation pathway, to date known as an important nutrient sensor only in eukaryotes. Up-regulation of a gene encoding the rate-limiting enzyme in the hexosamine pathway suggests a regulatory role for protein glycosylation in Synechocystis under HL.
Highlights
We found that the ndhD3, ndhF3, and cupA genes encoding for proteins involved in high-affinity CO2 transport were up-regulated (Klughammer et al, 1999; Ohkawa et al, 2002)
We found that genes coding for proteases and HL-inducible proteins (HLIPs) were significantly up-regulated in response to HL
Our data show that several genes involved in the biosynthesis of peptidoglycan were down-regulated in response to HL (Fig. 4). These results suggest that increased synthesis of UDP-GlcNAc as inferred by upregulation of the Gln:Fru6-P amidotransferase (GFAT) gene is utilized by O-linked GlcNAc transferase (OGT) for the addition of GlcNAc to substrate proteins
Summary
Synechocystis cells were grown at 30°C in BG11 medium buffered with 10 mM TES-KOH (pH 8.2) and bubbled with air. Illumination was at 30 mE m22 s21 (LL) provided by fluorescent cool-white lights. Cell growth was monitored by measuring OD at 730 nm on a DW2000 (SLM-AMINCO). For HL treatment, cells grown at LL were transferred in a long test tube (3 cm in diameter) to a cell density of approximately 5 3 107 cells/mL. The tubes containing cells were transferred in a thermostat water bath maintained at 30°C and illuminated with a white-light intensity of 300 mE m22 s21 (HL). Cells were air bubbled during HL treatment. Cells from LL- and HL-illuminated cultures were collected after 15 min, 1 h, 2 h, 3 h, 4 h, and 6 h. Cells were harvested by centrifugation at 6,000g, frozen in liquid nitrogen, and stored at 280°C
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