Altered carbon metabolism results in increased nonpolar lipid production in Synechocystis sp. PCC 6803

Synechocystis sp. PCC 6803 is a model cyanobacterium which is amenable to genetic manipulation, owing to its well characterized genome, short generation time and transformation efficiency. Under stress like nitrogen limitation, Synechocystis undergoes metabolic reprogramming, including the accumulation of carbon reserves in the form of glycogen and polyhydroxybutyrate (PHB). While historically thought to lack triacylglycerol (TAG) and wax esters, recent studies have confirmed the presence of TAG and fatty acid phytyl esters (FAPE) in Synechocystis and other cyanobacteria. TAG and FAPE represent vital forms of carbon and energy storage in plants and microalgae, but Synechocystis produces only minute amounts of these nonpolar lipids. We sought to boost their yields by blocking competing pathways of glycogen and PHB synthesis. Under N replete conditions, the deletion of glycogen synthesis (Δpgm1 mutation) caused an increase in TAG and FAPE contents by 6.5-fold and 13.6-fold, respectively, while the inhibition of PHB synthesis ( ΔphaCE mutation) in combination with the mutation in the regulatory PirC protein (PII-interacting regulator of carbon metabolism ΔpirC ), resulted in increased pool sizes of TAG and FAPE by 5.7-fold and 2.4-fold, respectively. The regulatory mutant ΔpII revealed similar levels of TAG and FAPE, but the content of membrane lipids was strongly increased during N deprivation compared with wild type cells. These results show that additional amounts of carbon can be deposited in the form of TAG and wax esters in mutant strains with storage-sink blockage of glycogen and PHB.