Exponential-phase glycogen recycling is essential for growth of Mycobacterium smegmatis

Bacterial glycogen is a polyglucose storage compound that is thought to pro long viability during stationary phase. However, a specific role for glycog en has not been determined. We have characterized SMEG53, a temperature-sen sitive mutant of Mycobacterium smegmatis that contains a mutation in glgE, encoding a putative glucanase, This mutation causes exponentially growing S MEG53 cells to stop growing at 42 degrees C in response to high levels of g lycogen accumulation. The mutation in glgE is also associated with an alter ed growth rate and colony morphology at permissive temperatures; the severi ty of these phenotypes correlates with the amount of glycogen accumulated b y the mutant, Suppression of the temperature-sensitive phenotype, via a dec rease in glycogen accumulation, is mediated by growth in certain media or m ulticopy expression of garA, The function of GarA is unknown, but the prese nce of a forkhead-associated domain suggests that this protein is a member of a serine-threonine kinase signal transduction pathway. Our results sugge st that in M, smegmatis glycogen is continuously synthesized and then degra ded by GlgE throughout exponential growth, In turn, this constant recycling of glycogen controls the downstream availability of carbon and energy. Thu s, in addition to its conventional storage role, glycogen may also serve as a carbon capacitor for glycolysis during the exponential growth of M, smeg matis.