Several studies have investigated abundance-controlled regulation by tracking changes in the cyanobacterial transcriptome and proteome across the day-night cycle. The flux through an enzyme is regulated by changing its abundance, product/substrate concentration, or through post-translational effects that alter its apparent kinetic parameters. Metabolic shifts that occur at specific time points over the day-night cycle are governed by regulating the flux through key enzymes and pathways. During the night, CO 2 fixation and most biosynthetic pathways are inactive while glycogen is degraded to support cellular maintenance and a small subset of pathways that prepare the cell for the next light period ( Saha et al., 2016 Reimers et al., 2017 Welkie et al., 2019 Werner et al., 2019). During the day, CO 2 is fixed in the Calvin cycle and converted into biomass, including storage compounds such as glycogen. As their energy source is limited to the light hours of the day, cyanobacteria have evolved to shift between photosynthetic and respiratory metabolism between day and night, respectively. Knowledge of cyanobacterial metabolism and its regulation can guide metabolic engineering efforts to create more efficient strains for renewable fuel and chemical production. Identification and manipulation of such mechanisms could be part of a metabolic engineering strategy for overproduction of chemicals. In conclusion, these results suggest that cyanobacteria have evolved to govern diurnal metabolic shifts through allosteric regulatory mechanisms in order to avoid the energy burden of replacing the proteome on a daily basis. Instead, model simulations showed that these observations can be attributed to a slow protein turnover, which reduces the effect of protein synthesis oscillations on the protein level. Our experimental analysis revealed that protein oscillations were not dampened by translational regulation, as evidenced by high correlation between translational and transcriptional oscillations ( r = 0.88) and unchanged protein levels. Furthermore, the effect of protein turnover on the amplitude of protein oscillations was investigated through in silico simulations using a protein mass balance model. PCC 6803 was cultivated in an artificial day-night setting and the level of transcription, translation and protein was measured across the genome at different time points using mRNA sequencing, ribosome profiling and quantitative proteomics. To determine whether translational regulation counteracts transcriptional changes, Synechocystis sp. The aim of this study was to find the cause for the previously reported inconsistency between oscillating transcription and constant protein levels under day-night growth conditions. Yet, better understanding of metabolic regulation in cyanobacteria is required to develop more productive strains that can make industrial scale-up economically feasible. Metabolically engineered cyanobacteria have the potential to mitigate anthropogenic CO 2 emissions by converting CO 2 into renewable fuels and chemicals. 2Science for Life Laboratory, Stockholm, Sweden.1Department of Protein Science, KTH Royal Institute of Technology, Stockholm, Sweden.Jan Karlsen 1,2, Johannes Asplund-Samuelsson 1,2†, Michael Jahn 1,2†, Dóra Vitay 1,2,3† and Elton P.
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