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

Abstract

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 Galdieria sulphuraria belonging to the Cyanidiales. We hypothesized that two candidate genes, Gasu_10960 and Gasu_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 Gasu_10960 synthesized small amounts of isofloridoside, whereas the Gasu_26940 translation mix also produced small amounts of floridoside. Moreover, the expression of Gasu_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 Gasu_26940 and Gasu_10960 genes of G. sulphuraria encode the enzymatically active floridoside and isofloridoside phosphate synthase/phosphatase fusion proteins, respectively, crucial for salt acclimation.

The Transition from Glycogen to Starch Metabolism in Cyanobacteria and Eukaryotes

“…Indeed, UDP-Glc serves as substrate for many enzymes, like trehalose-phosphate synthase, galactose-1-phosphate uridylyltransferase, or UDP-galactose-4-epimerase (Lluisma and Ragan 1998). This nucleotide sugar is involved in multiple biosynthetic pathways such as cell wall, isofloridoside, and floridoside synthesis (Pade et al 2014). Interestingly, floridoside shares functions analogous to those of sucrose in plants: it is an osmoprotectant, potentially used to translocate carbon between cells, and represents a major soluble source of storage carbon (Ekman et al 1991).…”

Heterosides – compatible solutes occurring in prokaryotic and eukaryotic phototrophs

“…Only recently, the first genes coding for proteins showing IFPS and FPS activity were identified in the extremophilic, unicellular red alga Galdieria sulphuraria (Pade et al . ). This alga can grow under many different environmental conditions, including widely varying salinities.…”

“…This starch‐binding domain might associate the protein with floridean starch granules in the red algal cell and support access to UDP‐glucose released during starch breakdown (Pade et al . ). The calculated molecular masses of the candidate proteins are around 120 kDa, which corresponds well to the molecular masses of FPS or IFPS proteins previously detected in algal extracts (discussed above).…”

“…Further details regarding methodology can be found in Pade et al . (). The proteins involved in (iso)floridoside synthesis from Galdieria sulphuraria ( FPS and IFPS ) are shown in bold.…”

“…The first candidate protein, Gasu_10960, was able to produce isofloridoside from these precursors (Pade et al . ). This result revealed that gasu_10960 codes for a fusion protein catalysing the IFPS as well as the isofloridoside 3‐phosphate phosphatase reaction.…”