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

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 ({Delta}sll0944-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.