Carbon/nitrogen homeostasis control in cyanobacteria

Abstract

Carbon/nitrogen (C/N) balance sensing is a key requirement for the maintenance of cellular homeostasis. Therefore, cyanobacteria have evolved a sophisticated signal transduction network targeting the metabolite 2-oxoglutarate (2-OG), the carbon skeleton for nitrogen assimilation. It serves as a status reporter for the cellular C/N balance that is sensed by transcription factors NtcA and NdhR and the versatile PII-signaling protein. The PII protein acts as a multitasking signal-integrating regulator, combining the 2-OG signal with the energy state of the cell through adenyl-nucleotide binding. Depending on these integrated signals, PII orchestrates metabolic activities in response to environmental changes through binding to various targets. In addition to 2-OG, other status reporter metabolites have recently been discovered, mainly indicating the carbon status of the cells. One of them is cAMP, which is sensed by the PII-like protein SbtB. The present review focuses, with a main emphasis on unicellular model strains Synechoccus elongatus and Synechocystis sp. PCC 6803, on the physiological framework of these complex regulatory loops, the tight linkage to metabolism and the molecular mechanisms governing the signaling processes.

On the nature of thiamine triphosphate in Arabidopsis

“…Both of these enzymes are involved (directly or indirectly) in amino acid biosynthesis. GDH is required for conversion of glutamate to α-ketoglutarate, which is a key molecule that integrates carbon and nitrogen metabolism (Forchhammer & Selim, 2020). On the other hand, PLK contributes to the production of pyridoxal 5'-phosphate, a coenzyme required for numerous enzymes predominantly involved in amino acid metabolism (Fitzpatrick, 2011).…”

The Novel PII-Interacting Regulator PirC (Sll0944) Identifies 3-Phosphoglycerate Mutase (PGAM) as Central Control Point of Carbon Storage Metabolism in Cyanobacteria

“…The PII proteins act as multitasking signal integrators that combine information on the metabolic C/N balance through interaction with the status reporter metabolite 2-oxoglutarate (2-OG) and the energy state of the cell by competitive binding of ATP or ADP. 2-OG is ideally suited as a status reporter metabolite for C/N balance, as this tricarboxylic acid (TCA) cycle intermediate represents the precursor metabolite into which nitrogen in form of ammonia is incorporated through the assimilatory reactions catalysed by the glutamine synthetaseglutamate synthase (GS/GOGAT) cycle (3).…”

“…The interaction of PII proteins with various effector molecules, as well as how they transmit these information into conformational states that are perceived by the targets, has been elaborated in great detail (recently reviewed (3)(4)(5)(6)). The three intersubunit clefts of the trimeric PII proteins contain intercommunicating effector molecule binding sites: there, ADP or ATP compete for occupying these sites, whereby ATP creates a ligation sphere for the effector 2-OG through a bridging Mg 2+ ion.…”

Expression of Formate-Tetrahydrofolate Ligase Did Not Improve Growth but Interferes With Nitrogen and Carbon Metabolism of Synechocystis sp. PCC 6803

“…The activity of the transcriptional factor NtcA is strongly regulated by the PII protein (glnB; 0.6-fold expression in exFTL, Table 2), which in dependence on the amount of 2OG and ATP regulates NtcA activity via an adaptor protein PipX. For example, at high N/C ratios with a lowered 2OG level, PII efficiently binds to PipX and NtcA free of PipX is inactivated, while under low N/C ratios with high 2OG amounts, NtcA-2OG is activated due to strong PipX-binding (Forchhammer and Selim, 2020). According to this model, the decreased 2OG level in exFTL indicates a relative nitrogen-rich status in exFTL (Forchhammer and Lüddecke, 2016).…”