Integration of carbon and nitrogen metabolism with energy production is crucial to light acclimation in the cyanobacterium Synechocystis

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.

Figure 1.

A, Schematic diagram showing events following absorption of light by PSII. The excitation energy absorbed by pigments associated with PSII is released primarily via three routes, either as heat, fluorescence, or utilized for photochemistry. B, Time course of changes in F_v/F_m ratios following HL treatment. Synechocystis cells grown at LL (30 μE m⁻² s⁻¹) were transitioned to HL (300 μE m⁻² s⁻¹). Temperature was maintained at 30°C during growth and treatment. Samples were withdrawn at indicated time period and fluorescence was measured in FL100.

Figure 2.

Distinct groups of differentially regulated genes in response to HL as revealed by clustering. The differentially regulated genes were clustered using discretized expressions as described in “Materials and Methods.” The characteristics of each cluster are described in the text and genes present in various clusters are provided in Supplemental Table S3 along with their functional roles and fold change at each time point. The two solid lines in each cluster represent the fold-change cutoff (+0.3785 to −0.3785).

Figure 3.

Summary of differentially regulated genes belonging to selected processes in response to HL. Regulation pattern of genes belonging to a given process is identified by the cluster numbers as identified in Figure 2. The number of genes present in each cluster is provided in the bracket. Red and green colors represent up- and down-regulation of genes, respectively. Details of differentially regulated genes are provided in Supplemental Table S3.

Figure 4.

Schematic diagram of the HS pathway in Synechocystis. Genes involved in the pathway have been listed along with their regulation by HL treatment.

Figure 5.

A, RT-PCR of the GFAT gene. Synechocystis cells were treated with HL for indicated time periods. Total RNAs were isolated and RT-PCR was performed as described in “Materials and Methods.” B and C, Coomassie-stained SDS-polyacrylamide gel (B) and western-blot analysis (C) of total cell extracts isolated from Synechocystis grown under various light conditions using CTD110.6 against O-GlcNAc. Synechocystis cells were grown in BG11 medium at 5 μE m⁻² s⁻¹ (lane 1); 30 μE m⁻² s⁻¹ (lane 2); and 200 μE m⁻² s⁻¹ for 24 h (lane 3).

Figure 6.

Schematic model of HL response in Synechocystis. Black arrows indicate the sequence in which HL impacts metabolic pathways. Broken arrows indicate the integration of responses that have interdependent impact on each other.