Improved sorbitol production and growth in cyanobacteria using promiscuous haloacid dehalogenase-like hydrolase

Sorbitol can be produced photosynthetically in the model cyanobacterium Synechocystis sp. PCC 6803 harboring sorbitol-6-phosphate (S6P) dehydrogenase (S6PDH) from apple, representing a promising metabolic engineering strategy for environmentally friendly carbon-based compound production. However, no gene for S6P phosphatase has been reported to date. We previously found that members of the Escherichia coli haloacid dehalogenase-like hydrolase (HAD) superfamily, a group of promiscuous hydrolases, exert phosphatase activity to S6P in vitro. Here, we examined the effects of HADs on sorbitol production in cyanobacterial cells. Overexpression of E. coli HAD1 improved sorbitol production induced by toxic S6PDH, whereas it suppressed growth even without induction. Moreover, overexpression of HAD1 in combination with engineering of other pathways successfully allowed for the constitutive expression of toxic S6PDH. Consequently, the sorbitol production was highly improved, which in turn suppressed the growth suppression effect of promiscuous HAD1. These results provide a good example of a novel metabolic engineering strategy using a combination of a promiscuous enzyme with an abundant supply of one of its substrates.