Abstract:
Glycogen is a multibranched glucose polymer and the major carbon storage compound in most free-living bacteria. In cyanobacteria glycogen metabolism is tightly coupled to a photosynthetic lifestyle where it is built up during photoautotrophic growth and degraded during heterotrophic phases. Next to being essential for diurnal growth, glycogen is also crucial during nutrient starvation, where the deprivation of nitrogen is the most common form for non-diazotrophic cyanobacteria. When encountering nitrogen starvation, the unicellular cyanobacterium Synechocystis sp. PCC6803 enters a dormant like state, called chlorosis, enabling it to outlive long starvation periods from which it can resuscitate upon re-availability of combined nitrogen sources. These metabolic adaptations are dependent on the rapid synthesis or breakdown of glycogen and must be precisely regulated. Like many other cyanobacteria, the genome of Synechocystis encodes for two isoforms for most of the enzymes involved in glycogen turnover. Despite the great significance that glycogen metabolism has in cyanobacteria, the role and regulation of many of these enzymes is only poorly understood. In this study the regulation of glycogen metabolism and its associated enzymes were investigated in detail. Glycogen degradation is performed by the coordinated action of glycogen phosphorylases (GlgP) and glycogen debranching enzymes (GlgX). This study revealed that GlgP1 is under the control of a C-terminal redox switch and activated upon oxidation. This connects GlgP1 activity to stress conditions that lead to the formation of reactive oxygen species. Furthermore, this study showed for the first time that GlgX1 is the essential glycogen debranching enzyme during resuscitation from nitrogen starvation while the role of GlgX2 remains elusive. A central step in carbon metabolism is the interconversion of glucose phosphate by the phosphoglucomutase (PGM) reaction which is essential for glycogen synthesis and degradation alike. The PGM reaction is dependent on the availability of the central activator Glc-1,6-BP whose origin in prokaryotes was so far unknown. This work identified the PGM homologue Slr1334 as the first Glc-1,6-BP synthase. It was shown that Slr1334 homologues are widespread among prokaryotes and that a Slr1334 homologue from Bacteriodes salyersiae catalyzes the same reaction. Glc-1,6-BP represents a so far unconsidered regulator in bacterial carbon metabolism, which is suggested to regulate several key enzymes besides PGM.
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