The genome of the thermoacidophilic red microalga Galdieria sulphuraria encodes a small family of secreted class III peroxidases that might be involved in cell wall modification.

We report the identification of a small family of secreted class III plant peroxidases (Prx) from the genome of the unicellular thermoacidophilic red alga Galdieria sulphuraria (Cyanidiaceae). Apart from two class I ascorbate peroxidases and one cytochrome c peroxidase, the red algal genome encodes four class III plant peroxidases, thus complementing the short list of algal cell wall peroxidases (Passardi et al. in Genomics 89:567-579, 2007). We have characterized the family gene structure, analyzed the extracellular space and cell wall fraction of G. sulphuraria for the presence of peroxidase activity and used shotgun proteomics to identify candidate extracellular peroxidases. For a detailed enzymatic characterization, we have purified a secreted peroxidase (GsPrx04) from the cell-free medium using hydrophobic interaction chromatography. The enzyme proved heat and acid-stable and exhibited an apparent molecular mass of 40 kDa. Comparative genomics between endolithically growing G. sulphuraria and a close relative, the obligatory aquatic, cell wall-less Cyanidioschyzon merolae, revealed that class III peroxidases only occur in the terrestrial microalga, thus supporting the key function of these enzymes in the process of land colonization.

The reaction mechanism of phosphomannomutase in plants.

The enzyme phosphomannomutase catalyzes the interconversion of mannose-1-phosphate (Man-1-P) and mannose-6-phosphate (Man-6-P). In mammalian cells the enzyme has to be activated by transfer of a phosphate group from a sugar-1.6-P2 (Guha, S.K. and Rose, Z.B. (1985) Arch. Biochem. Biophys. 243, 168). In contrast, in the red alga Galdieria sulphuraria the co-substrate (Man-1.6-P2 or Glc-1.6-P2) is converted to the corresponding sugar monophosphate while the substrate is converted to the sugar bisphosphate in each reaction cycle. Evidence is presented that the same reaction mechanism occurs in spinach and yeast.