Metabolic regulation of carbon allocation from the CBB cycle under different CO2 supply

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Cyanobacteria are photoautotrophic organisms, which assimilate CO2 during the day and use accumulated organic carbon by heterotrophic pathways during the night. Furthermore, Synechocystis activates the carbon flow from the Calvin cycle in the direction of lower glycolysis and reduces glycogen reserves after shifting cells from high to low CO2 conditions, which indicates a change from autotrophic to heterotrophic metabolism in light. Hence, anabolic and catabolic routes of primary carbon metabolism are both occurring in the non- compartmentalized cell, which need to be tightly regulated to avoid futile cycles. We assume that isoenzymes at metabolic branch points between the Calvin cycle and glycolysis (e.g. phosphoglycerate mutases, PGAM, or glyceraldehyde-3-phosphate dehydrogenases, GapDH) are at least partially responsible for these changes.In the first project phase, we investigated the role of GapDHs and PGAMs with the help of specific mutants in the anabolic or catabolic carbon flow depending on the CO2 content. For the carbon flow in the direction of the lower glycolysis, PGAM1 is mainly responsible, the activity of which is switched on or off by the regulatory protein PirC, while the other PGAM isoenzymes tend not to be involved. Further experiments showed that GapDH1 cannot use NADPH2 and participates in heterotrophic metabolism. The GapDH2 is involved in the Calvin cycle and is mainly regulated by the protein CP12. Investigations of the phosphoproteome showed that CP12 and subunits of a bicarbonate transporter are dephosphorylated under low CO2 conditions, while the phosphorylation of the protein kinase SpkC increases. In addition, other protein kinases seem to be involved in the CO2 acclimation. Finally, metabolome studies on cells in a day / night rhythm indicated that RubisCO is also active at night.Based on these results, we want to focus our research in cooperation with other SCyCode groups on the following four points in the upcoming phase. 1. What role does CP12, in particular its differential phosphorylation, play in regulating the Calvin cycle activity in light? 2. Which protein kinases or phosphatases are responsible for the differential phosphorylation of proteins during the CO2 acclimation? 3. To what extent is RubisCO active in Synechocystis in the dark phase and is it regulated by di-phosphorylated sugars? 4. The existing data sets will finally be integrated into a kinetic model of the CO2 acclimation of the primary carbon metabolism together with a Mercator Fellow. These investigations should also make it possible to optimize the cyanobacterial carbon metabolism specifically for biotechnological applications in the future.