Synechocystis sp. PCC 6803 PG is a cyanobacterial strain capable of synthesizing 1,2-propanediol from carbon dioxide (CO2 ) via a heterologous three-step pathway and a methylglyoxal synthase (MGS) originating from Escherichia coli as an initial enzyme. The production window is restricted to the late growth and stationary phase and is apparently coupled to glycogen turnover. To understand the underlying principle of the carbon partitioning between the Calvin-Benson-Bassham (CBB) cycle and glycogen in the context of 1,2-propanediol production, experiments utilizing 13 C labeled CO2 have been conducted. Carbon fluxes and partitioning between biomass, storage compounds, and product have been monitored under permanent illumination as well as under dark conditions. About one-quarter of the carbon incorporated into 1,2-propanediol originated from glycogen, while the rest was derived from CO2 fixed in the CBB cycle during product formation. Furthermore, 1,2-propanediol synthesis was depending on the availability of photosynthetic active radiation and glycogen catabolism. We postulate that the regulation of the MGS from E. coli conflicts with the heterologous reactions leading to 1,2-propanediol in Synechocystis sp. PCC 6803 PG. Additionally, homology comparison of the genomic sequence to genes encoding for the methylglyoxal bypass in E. coli suggested the existence of such a pathway also in Synechocystis sp. PCC 6803. These findings are critical for all heterologous pathways coupled to the CBB cycle intermediate dihydroxyacetone phosphate via a MGS and reveal possible engineering targets for rational strain optimization.
Keywords: 1,2-propanediol; Synechocystis sp. PCC 6803; carbon flux; cyanobacteria; glycogen; methylglyoxal synthase.
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