Increased Biomass and Polyhydroxybutyrate Production by Synechocystis sp. PCC 6803 Overexpressing RuBisCO Genes

Abstract

The overexpression of the <i>RuBisCO</i> (<i>rbc</i>) gene has recently become an achievable strategy for increasing cyanobacterial biomass and overcoming the biocompound production restriction. We successfully constructed two <i>rbc</i>-overexpressing <i>Synechocystis</i> sp. PCC 6803 strains (OX), including a strain overexpressing a large subunit of RuBisCO (OX<i>rbcL</i>) and another strain overexpressing all large, chaperone, and small subunits of RuBisCO (OX<i>rbcLXS</i>), resulting in higher and faster growth than wild type under sodium bicarbonate supplementation. This increased biomass of OX strains significantly contributed to the higher polyhydroxybutyrate (PHB) production induced by nutrient-deprived conditions, in particular nitrogen (N) and phosphorus (P). As a result of higher PHB contents in OX strains occurring at days 7 and 9 of nutrient deprivation, this enhancement was apparently made possible by cells preferentially maintaining their internal lipids while accumulating less glycogen. The OX<i>rbcLXS</i> strain, with the highest level of PHB at about 39 %w/dry cell weight (DCW) during 7 days of BG₁₁-NP treatment, contained a lower glycogen level (31.9 %w/DCW) than wild type control (40 %w/DCW). In contrast, the wild type control strain exposed to N- and NP-stresses tended to retain lipid levels and store more glycogen than PHB. In this model, we, for the first time, implemented a <i>RuBisCO</i>-overexpressing cyanobacterial factory for overproducing PHB, destined for biofuel and biomaterial biotechnology.