Stress response requires an efficient connection between glycogen and central carbon metabolism by phosphoglucomutases in cyanobacteria

Glycogen and starch are the main storage polysaccharides, acting as a source of carbon and energy when necessary. Interconversion of glucose-1-phosphate and glucose-6-phosphate by phosphoglucomutases connects the metabolism of these polysaccharides with central carbon metabolism. However, knowledge about how this connection affects the ability of cells to cope with environmental stresses is still scarce. The cyanobacterium Synechocystis sp. PCC 6803 has two enzymes with phosphoglucomutase activity, PGM (phosphoglucomutase) and PMM/PGM (phosphomannomutase/phosphoglucomutase). In this work, we generated a null mutant of PGM ( increment PGM) that exhibits very reduced phosphoglucomutase activity (1% of wild type activity). Although this mutant accumulates moderate amounts of glycogen, its phenotype resembles that of glycogen-less mutants, including high light sensitivity and altered response to nitrogen deprivation. Using an on/off arsenite promoter, we demonstrate that PMM/PGM is essential for growth and responsible for the remaining phosphoglucomutase activity in the increment PGM strain. Furthermore, overexpression of PMM/PGM in the increment PGM strain is enough to revoke the phenotype of this mutant. These results emphasize the importance of an adequate flux between glycogen and central carbon metabolism to maintain cellular fitness and indicate that although PGM is the main phosphoglucomutase activity, the phosphoglucomutase activity of PMM/PGM can substitute it when expressed in sufficient amounts. Synechocystis has two enzymes with phosphoglucomutase activity: PGM and PMM/PGM. PGM is responsible for most of the phosphoglucomutase activity, but PMM/PGM is sufficient to sustain glycogen metabolism and its overexpression overrides the stress sensitivity phenotype of a PGM-null mutant.

Automated summary

Glycogen and starch are important polysaccharides for carbon and energy storage. The interconversion of glucose-1-phosphate and glucose-6-phosphate by phosphoglucomutases connects the metabolism of these polysaccharides with central carbon metabolism. The enzyme PMM/PGM plays a crucial role in supporting glycogen metabolism and its overexpression can overcome stress sensitivity in a PGM-null mutant.