Background PHB (poly-hydroxy-butyrate) represents a promising bioplastic alternative with good biodegradation properties. Furthermore, PHB can be produced in a completely carbon–neutral fashion in the natural producer cyanobacterium Synechocystis sp. PCC 6803. This strain has been used as model system in past attempts to boost the intracellular production of PHB above ~ 15% per cell-dry-weight (CDW). Results We have created a new strain that lacks the regulatory protein PirC (product of sll0944), which exhibits a higher activity of the phosphoglycerate mutase resulting in increased PHB pools under nutrient limiting conditions. To further improve the intracellular PHB content, two genes involved in PHB metabolism, phaA and phaB, from the known producer strain Cupriavidus necator, were introduced under the control of the strong promotor PpsbA2. The resulting strain, termed PPT1 (ΔpirC-REphaAB), produced high amounts of PHB under continuous light as well under a day-night regime. When grown in nitrogen and phosphorus depleted medium, the cells produced up to 63% per CDW. Upon the addition of acetate, the content was further increased to 81% per CDW. The produced polymer consists of pure PHB, which is highly isotactic. Conclusion The amounts of PHB achieved with PPT1 are the highest ever reported in any known cyanobacterium and demonstrate the potential of cyanobacteria for a sustainable, industrial production of PHB.

Nitrogen limitation imposes a major transition in the lifestyle of nondiazotrophic cyanobacteria that is controlled by a complex interplay of regulatory factors involving the pervasive signal processor PII. Immediately upon nitrogen limitation, newly fixed carbon is redirected toward glycogen synthesis. How the metabolic switch for diverting fixed carbon toward the synthesis of glycogen or of cellular building blocks is operated was so far poorly understood. Here, using the nondiazotrophic cyanobacterium Synechocystis sp. PCC 6803 as model system, we identified a novel PII interactor, the product of the sll0944 gene, which we named PirC. We show that PirC binds to and inhibits the activity of 2,3-phosphoglycerate–independent phosphoglycerate mutase (PGAM), the enzyme that deviates newly fixed CO2 toward lower glycolysis. The binding of PirC to either PII or PGAM is tuned by the metabolite 2-oxoglutarate (2-OG), which accumulates upon nitrogen starvation. In these conditions, the high levels of 2-OG dissociate the PirC–PII complex to promote PirC binding to and inhibition of PGAM. Accordingly, a PirC-deficient mutant showed strongly reduced glycogen levels upon nitrogen deprivation, whereas polyhydroxybutyrate granules were overaccumulated compared to wild-type. Metabolome analysis revealed an imbalance in 3-phosphoglycerate to pyruvate levels in the pirC mutant, confirming that PirC controls the carbon flux in cyanobacteria via mutually exclusive interaction with either PII or PGAM.

Phosphoenolpyruvate carboxylase (PEPC) is the second major carbon‐fixing enzyme in photoautotrophic organisms. PEPC is required for the synthesis of amino acids of the glutamate and aspartate family by replenishing the TCA cycle. Furthermore, in cyanobacteria, PEPC, together with malate dehydrogenase and malic enzyme, forms a metabolic shunt for the synthesis of pyruvate from PEP. During this process, CO2 is first fixed and later released again. Due to its central metabolic position, it is crucial to fully understand the regulation of PEPC. Here, we identify PEPC from the cyanobacterium Synechocystis sp. PCC 6803 (PEPC) as a novel interaction partner for the global signal transduction protein PII. In addition to an extensive characterization of PEPC, we demonstrate specific PII–PEPC complex formation and its enzymatic consequences. PEPC activity is tuned by the metabolite‐sensing properties of PII: Whereas in the absence of PII, PEPC is subjected to ATP inhibition, it is activated beyond its basal activity in the presence of PII. Furthermore, PII–PEPC complex formation is inhibited by ADP and PEPC activation by PII‐ATP is mitigated in the presence of 2‐OG, linking PEPC regulation to the cell's global carbon/nitrogen status. Finally, physiological relevance of the in vitro measurements was proven by metabolomic analyses of Synechocystis wild‐type and PII‐deficient cells.

Nitrogen limitation imposes a major transition in the life-style of non-diazotrophic cyanobacteria, which is regulated via a complex interplay of regulatory factors, involving, the nitrogen-specific transcription factor NtcA and the pervasive signal processor PII. Immediately upon nitrogen-limitation, newly fixed carbon is re-directed towards glycogen synthesis. How the metabolic switch for distributing fixed carbon to either glycogen or cellular building blocks is operated was poorly understood. Here we identify from Synechocystis sp. PCC 6803 a novel PII interactor, PirC, (Sll0944) that controls 3-phosphoglycerate mutase (PGAM), the enzyme that deviates newly fixed CO2 towards lower glycolysis. PirC acts as competitive inhibitor of PGAM and this interaction is tuned by PII/2-oxoglutarate. High oxoglutarate release PirC from PII-complex to inhibit PGAM. Accordingly, PirC deficient mutant, as compared to the wild-type, shows strongly reduced glycogen levels upon nitrogen deprivation whereas polyhydroxybutyrate granules are over-accumulated. Metabolome analysis revealed an imbalance in 3-phosphoglycerate to pyruvate levels in the PirC mutant, conforming that PirC controls the carbon flux in cyanobacteria via mutually exclusive interaction with either PII or PGAM.