Floridoside and isofloridoside are synthesized by trehalose 6‐phosphate synthase‐like enzymes in the red alga G aldieria sulphuraria

Summary Compatible solutes are small molecules that are involved in acclimation to various abiotic stresses, especially high salinity. Among the red algae, the main photosynthetic products floridoside and isofloridoside (galactosylglycerols) are known also to contribute to the osmotic acclimation of cells. However, the genes encoding (iso)floridoside biosynthetic enzymes are still unknown. To identify candidate genes, we examined the genome of the floridoside‐ and isofloridoside‐accumulating extremophilic red alga G aldieria sulphuraria belonging to the C yanidiales. We hypothesized that two candidate genes, G asu_10960 and G asu_26940 , code for enzymes involved in floridoside and isofloridoside biosynthesis. These proteins comprise a sugar phosphate synthase and a sugar phosphate phosphatase domain. To verify their biochemical activity, both genes were in vitro translated into the entire proteins. The protein translation mixture containing G asu_10960 synthesized small amounts of isofloridoside, whereas the G asu_26940 translation mix also produced small amounts of floridoside. Moreover, the expression of G asu_10960 in a salt‐sensitive mutant of the cyanobacterium Synechocystis sp. PCC 6803 resulted in increased salt tolerance as a consequence of the presence of isofloridoside in the complemented cells. Thus, our experiments suggest that the G asu_26940 and G asu_10960 genes of G . sulphuraria encode the enzymatically active floridoside and isofloridoside phosphate synthase/phosphatase fusion proteins, respectively, crucial for salt acclimation.