Adaptive laboratory evolution of a mannitol producing Synechocystis PCC6803 derivative to salt stress

Mannitol is a naturally occurring polyol, with a wide range of applications. In a previous study, mannitol production was achieved via the direct conversion of CO2 in Synechocystis PCC6803. However, a major barrier to the industrial application of this strain is its low production rate of mannitol. In this study, three mutants were isolated after 164 generations of adaptive laboratory evolution under salt stress. These mannitol overproducing mutants were able to produce 27.71 mg per liter per OD730 under 350 mM salt stress when OD730 reached 2, roughly 24 times higher than their parental strains. Whole genome resequencing was then performed, revealing mutations in 2 genes, pnp and sigA/rpoD1, of the overproducing mutants when compared to the parental strain. Of these genes, pnp which encodes for polyribonucleotide nucleotidyltransferase, was found to negatively affect mannitol production in cells via reverse engineering methods, since removal of pnp alone resulted in 6.4-fold increase in the mannitol titer.