AUTHOR=Lucius Stefan , Makowka Alexander , Michl Klaudia , Gutekunst Kirstin , Hagemann Martin 

TITLE=The Entner-Doudoroff Pathway Contributes to Glycogen Breakdown During High to Low CO2 Shifts in the Cyanobacterium Synechocystis sp. PCC 6803

JOURNAL=Frontiers in Plant Science

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2021.787943

DOI=10.3389/fpls.2021.787943

ISSN=1664-462X

ABSTRACT=<p>Cyanobacteria perform plant-like oxygenic photosynthesis to convert inorganic carbon into organic compounds and can also use internal carbohydrate reserves under specific conditions. A mutant collection with defects in different routes for sugar catabolism was studied to analyze which of them is preferentially used to degrade glycogen reserves in light-exposed cells of <italic>Synechocystis</italic> sp. PCC 6803 shifted from high to low CO<sub>2</sub> conditions. Mutants defective in the glycolytic Embden–Meyerhof–Parnas pathway or in the oxidative pentose-phosphate (OPP) pathway showed glycogen levels similar to wild type under high CO<sub>2</sub> (HC) conditions and were able to degrade it similarly after shifts to low CO<sub>2</sub> (LC) conditions. In contrast, the mutant Δ<italic>eda</italic>, which is defective in the glycolytic Entner-Doudoroff (ED) pathway, accumulated elevated glycogen levels under HC that were more slowly consumed during the LC shift. In consequence, the mutant Δ<italic>eda</italic> showed a lowered ability to respond to the inorganic carbon shifts, displayed a pronounced lack in the reactivation of growth when brought back to HC, and differed significantly in its metabolite composition. Particularly, Δ<italic>eda</italic> accumulated enhanced levels of proline, which is a well-known metabolite to maintain redox balances via NADPH levels in many organisms under stress conditions. We suggest that deletion of <italic>eda</italic> might promote the utilization of the OPP shunt that dramatically enhance NADPH levels. Collectively, the results point at a major regulatory contribution of the ED pathway for the mobilization of glycogen reserves during rapid acclimation to fluctuating CO<sub>2</sub> conditions.</p>