Glycogen phosphorylase and phosphoenolpyruvate synthase are key metabolic nodes for high-light acclimation in cyanobacteria

Balancing energy supply from photosynthetic light reactions with downstream metabolic demand is critical for preventing photosystem damage. Although energy-dissipation mechanisms involving remodeling of photosystems are well known, the role of downstream metabolic routes in energy dissipation remains understudied. Here, we show that two metabolic nodes, glycogen phosphorylase (glgP) and phosphoenolpyruvate (PEP) synthase (pps), participate in photoprotection under high light in Synechococcus elongatus PCC 7942. Glycogen phosphorylase is important for photosystem I (PSI) protection under high or changing light conditions. Under high light, the ΔglgP mutant exhibits severe chlorosis and reduced cyclic electron flow (CEF), approximated from the P700+ re-reduction rate, without major changes in carbon metabolism. Comparative proteomics further suggests redox exchange from NADPH to NADH, consistent with a role of GlgP in maintaining P700 acceptor side oxidized via enhanced CEF. Meanwhile, PEP synthase acts as a key node regulating tricarboxylic acid (TCA) cycle flux and potentially NADPH production. Under high light, the Δpps mutant rewires its metabolism to enhance CEF, likely a compensatory mechanism that offsets excess NADPH from elevated TCA flux. Long-term maintenance of Δpps resulted in a spontaneous secondary deletion in adhE, encoding succinate semialdehyde dehydrogenase (SSADH) of the TCA bypass. This suppressor mutation, combined with the loss of PEP synthase, partially mitigates high-light stress at the cost of reduced amino acid synthesis. These findings reveal important roles for downstream carbon metabolism in high-light acclimation and provide new insights into photoprotective mechanisms in phototrophs.