Improvement of 1,3-propanediol production using an engineered cyanobacterium, Synechococcus elongatus by optimization of the gene expression level of a synthetic metabolic pathway and production conditions

- The highest titer of 1,3-PDO (16.1 mM, 1.22 g/l) was achieved by cyanobacteria.
- The sum of products (1,3-PDO and glycerol) derived from carbon dioxide reached 25.5 mM.
- Promoter replacement increased the conversion rate of glycerol into 1,3-PDO.
- The titer of 1,3-PDO was increased by growth phase elongation.

The introduction of a synthetic metabolic pathway consisting of multiple genes derived from various organisms enables cyanobacteria to directly produce valuable chemicals from carbon dioxide. We previously constructed a synthetic metabolic pathway composed of genes from Escherichia coli, Saccharomyces cerevisiae, and Klebsiella pneumoniae. This pathway enabled 1,3-propanediol (1,3-PDO) production from cellular DHAP via glycerol in the cyanobacterium, Synechococcus elongatus PCC 7942. The production of 1,3-PDO (3.79 mM, 0.29 g/l) directly from carbon dioxide by engineered S. elongatus PCC 7942 was successfully accomplished. However, the constructed strain accumulated a remarkable amount of glycerol (12.6 mM, 1.16 g/l), an intermediate metabolite in 1,3-PDO production. Notably, enhancement of latter reactions of synthetic metabolic pathway for conversion of glycerol to 1,3-PDO increases 1,3-PDO production. In this study, we aimed to increase the observed 1,3-PDO production titer. First, the weaker S. elongatus PCC 7942 promoter, PLlacO1, was replaced with a stronger promoter (Ptrc) to regulate genes involved in the conversion of glycerol to 1,3-PDO. Second, the induction timing for gene expression and medium composition were optimized. Promoter replacement resulted in higher 1,3-PDO production than glycerol accumulation, and the amount of products (1,3-PDO and glycerol) generated via the synthetic metabolic pathway increased with optimization of medium composition. Accordingly, we achieved the highest titer of 1,3-PDO (16.1 mM, 1.22 g/l) and this was higher than glycerol accumulation (9.46 mM, 0.87 g/l). The improved titer was over 4-fold higher than that of our previous study.