Energy and carbon metabolism during the awakening from metabolic dormancy in cyanobacteria

Abstract

Bacterial dormancy plays a crucial role in the survival and spread of bacterial populations. The capability of resuming growth after a dormant period allows bacterial cells to survive in an ever-changing environment. Cyanobacteria represent a diverse group of prokaryotes with an exceptional ability to adapt to different environmental conditions. One of the most common challenges cyanobacteria face in nature is nitrogen limitation. When the unicellular cyanobacterium Synechocystis sp. PCC 6803 lacks a source of combined nitrogen, cells undergo a metabolic adaptation that leads to a dormant state that allows them to survive these conditions for a prolonged period of time. This adaptation follows a genetically determined program and involves the degradation of most of the thylakoid membranes and the synthesis of glycogen stores. In the quiescent state, proper control of energy homeostasis and glycogen metabolism are essential for survival. In the present study, the regulation of the energy and carbon metabolism during nitrogen starvation was investigated. Dormant cells were shown to rely on a different mechanism of ATP synthesis than vegetative cells. During vegetative growth most of the cellular ATP is produced by the ATP synthases in a reaction that requires an electrochemical proton gradient across the thylakoid membrane, which is generated by photosynthetic or respiratory electron transport. In nitrogen-starved cells, the number of thylakoid membranes is very reduced, which implies a reduced capacity of thylakoidal ATP synthesis. Under these circumstances, cells rely on the ATP synthases located in the cytoplasmic membrane and on an extracellular electrochemical sodium gradient for ATP synthesis. This study unraveled the transient utilization of a sodium-motive force for energy generation as a survival strategy in response to adverse environmental conditions. Addition of a nitrogen source to dormant cells initiates the resuscitation program. Nitrogen assimilation triggers glycogen degradation, which provides the necessary energy and metabolic intermediates to regenerate the degraded cellular components. This work revealed that glycogen catabolism is induced by dephosphorylation and activation of phosphoglucomutase 1 (Pgm1), which acts as a metabolic valve to avoid premature usage of the glycogen stores before a nitrogen source is available. Remarkably, this regulatory mechanism seems to be evolutionary conserved. Only a specific glycogen mobilization strategy was shown to enable successful resuscitation, which involves the glycogen phosphorylase GlgP2, the oxidative pentose phosphate (OPP) pathway and the Enter- Doudoroff (ED) pathway. Furthermore, OPP cycle protein (OpcA) and glucose-6- phosphate dehydrogenase (G6PDH), the proteins involved in the first reaction of the OPP and ED pathways, were shown to interact with Pgm1 during recovery. These interactions might result in the formation of a metabolon that directs the carbon flux into the OPP and ED pathways to ensure effective awakening from dormancy.