Increased acetate production in Synechocystis sp. PCC 6803 strain engineered with an operon of phosphoketolase and phosphotransacetylase and further overexpression of acetate kinase

BACKGROUND: Photosynthetic microorganisms, such as cyanobacteria, are promising candidates for sustainable production of chemicals. Photosynthesis is a unique process where light energy is used to convert CO2 into carbon metabolites that sustain the cell`s metabolism. One of these products is acetate, a chemical with various applications in industry. Metabolic engineering can be used to increase the titer of extracellular acetate in the unicellular cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis). RESULTS: Simultaneous expression of phosphoketolase (PK) and phosphotransacetylase (Pta) resulted in an enhanced acetate titer in Synechocystis cells (Roussou et al. Metab Eng 88:250-260) [1]. In the present study these two enzymes were expressed in different locus in the genome as well as expressed in the same locus organized as a single operon. The latter design reached higher acetate production. Attempts to further optimize the production through the creation of fused protein did not result in significant higher values than 2.3 g/L previously reported. However, the production was further increased when acetate kinase (AckA) was additionally overexpressed. Cultivation of this strain in high density cultivation (CellDEG system) led to high levels of acetate with a maximum of 7.1 g/L cumulative acetate production after 12 days of experiment when the cultures were sampled every day. CONCLUSIONS: Synechocystis sp. PCC 6803 is a candidate for sustainable acetate production driven by sunlight and CO2. The high level of acetate production is result of combining genomic integration of heterogenous genes in the cell and overexpression of native genes through self-replication vector. The production level achieved through the high-density cultivation reveal the strain capabilities when the growth conditions are optimal.