Free fatty acid production in the cyanobacterium Synechocystis sp. PCC 6803 is enhanced by deletion of the cyAbrB2 transcriptional regulator.

The cyAbrB2 (Sll0822) transcriptional regulator in Synechocystis sp. PCC 6803 is involved in coordination of carbon and nitrogen metabolism and its deletion causes distinct phenotypes such as decreased expression levels of nitrogen-regulated genes and high accumulation of glycogen granules. From the viewpoint of metabolic engineering, the highly accumulated glycogen granules in the ΔcyabrB2 mutant could be a valuable source for the production of biofuels. Here, by disruption of the aas gene (slr1609) encoding acyl-acyl carrier protein synthetase and introduction of a gene encoding thioesterase from Umbellularia californica (UcTE), we conferred the ability of production and secretion of free fatty acids on the ΔcyabrB2 mutant. Notable features of the resulting ΔcyabrB2Δaas::UcTE strain compared with ΔcyabrB2 by RNA-seq analysis were decrease in expression levels of genes related to uptake and subsequent metabolism of nitrogen and carbon and increase in the expression level of sigE encoding a group 2 sigma factor. These changes in gene expression profile were not observed when the same genetic modification was introduced in the wild-type background. The ΔcyabrB2Δaas::UcTE strain showed two-folds higher free fatty acid productivity on a per OD730 basis compared with the Δaas::UcTE strain, without expense of the accumulated glycogen granules. This shows the potential of the ΔcyabrB2 mutant as the platform of biofuel production. The effective utilization of the accumulated glycogen must be the next task to be pursued.