Maximizing PHB content inSynechocystis sp.PCC 6803: development of a new photosynthetic overproduction strain

Abstract

PHB (poly-hydroxy-butyrate) represents a promising bioplastic variety with good biodegradation properties. Furthermore, PHB can be produced completely carbon-neutral when synthesized in the natural producer cyanobacterium Synechocystis sp. PCC 6803. This model strain has a long history of various attempts to further boost its low amounts of produced intracellular PHB of ~15 % per cell-dry-weight (CDW). We have created a new strain that lacks the regulatory protein PirC (gene product of sll0944), which causes a rapid conversion of the intracellular glycogen pools to PHB under nutrient limiting conditions. To further improve the intracellular PHB content, two genes from the PHB metabolism, phaA and phaB from the known production strain Cupriavidus necator, were introduced under the regime of the strong promotor PpsbA2. The created strain, termed PPT1 (Δsll0944-REphaAB), produced high amounts of PHB under continuous light as well under day-night rhythm. When grown in nitrogen and phosphor depleted medium, the cells produced up to 63 % / CDW. Upon the addition of acetate, the content was further increased to 81 % / CDW. The produced polymer consists of pure PHB, which is highly isotactic. The achieved amounts were the highest ever reported in any known cyanobacterium and demonstrate the potential of cyanobacteria for a sustainable, industrial production of PHB.

Trans-4-hydroxy-L-proline production by the cyanobacterium Synechocystis sp. PCC 6803

“…Furthermore, the pH is important for cellular processes like carbon uptake and the carbon concentrating mechanism ( Mangan et al., 2016 ; Price et al., 2002 ). However, unlike light intensity or nutrient deprivation, the pH is usually not considered as an important parameter when evaluating factors to increase the growth and bioproduction of cyanobacteria ( Hoschek et al., 2019 ; Koch et al., 2020 ; Ungerer et al., 2018 ). In an initial experiment using a 4 L flat-panel bio-reactor, it was noted that the daily production of Hyp was highest at a high pH of ~10, while at a lower pH the production decreased strongly ( Fig S5 ).…”

Dark blue-green: Cave-inhabiting cyanobacteria as a model for astrobiology

“…Interestingly, a high proportion of cave inhabiting cyanobacterial taxa have recently been shown to produce biocompatible and biodegradable bioplastics e.g., polyhydroxyalkanoates such as polyhydroxybutyrate (PHA / PHB; Figure 2H) with various application potential (Djebaili et al, 2022). Cyanobacteria have minimal nutrient requirements for growth and accumulate PHAs and PHBs through oxygenic photosynthesis under nutrient-limited conditions, such as nitrogen and/or phosphate starvation, where cells enter a quiescent state known as chlorosis (Koch et al, 2020a).…”

Polyhydroxybutyrate: A Useful Product of Chlorotic Cyanobacteria

“…Mutant strains of PirC show an increased flux of newly fixed carbon towards pyruvate through lower glycolysis, thereby providing the building blocks for enhanced PHB synthesis. In combination with the overexpression of heterologous phaAB genes, the mutant strain PPT1 was created [Koch et al, 2020b]. Under phototrophic conditions, this strain accumulated 63% PHB per cell dry weight.…”