Abstract:
Cyanobacteria rely on glycogen as their principal carbon and energy reserve to buffer Diel and
stress-induced fluctuations, yet the regulatory logic of glycogen metabolism in Synechocystis
sp. PCC 6803 has remained incompletely understood. This cumulative work dissects the
control architecture of glycogen synthesis and degradation by combining quantitative enzyme
kinetics, in vitro pathway reconstitution, kinetic modelling, proteomics, and physiological
analysis.
First, glucose-1-phosphate adenylyltransferase (GlgC) is characterised as a tightly regulated 3-
phosphoglycerate (3-PGA)/Pi ratio-sensing gate that converts changes in photosynthetic output
and phosphate availability into ADP-glucose supply, enforcing a strong Diel asymmetry of
glycogen synthesis. Second, the two glycogen synthases (GlgA1 and GlgA2) are shown to have
comparable intrinsic catalytic efficiencies but distinct operating regimes dictated by primer
architecture and branching. Together with the branching enzyme GlgB, they generate glycogen
particles with different chain-length distributions and branching patterns, revealing a division
of labour between throughput and architecture.
Third, glycogen catabolism is resolved into a redox- and stress-responsive module. GlgP2
emerges as the main glycogen phosphorylase under standard and nocturnal conditions, whereas
GlgP1 acts as a redox-controlled reserve activated under oxidising, stress-associated states.
Deep mobilisation of glycogen during prolonged darkness and resuscitation from chlorosis
critically depends on GlgX1-mediated debranching. Pulldown and co-immunoprecipitation
experiments further support a glycogen-centred protein neighbourhood linking GlgC–GlgA–
GlgB with GlgP/GlgX.
Together, these findings establish glycogen metabolism in Synechocystis as a spatially
organised, multi-layered control system that integrates light, phosphate, redox and nitrogen
signals, providing a mechanistic framework for rational engineering of cyanobacterial carbon
storage.
Glycogen