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Cell wall biosynthesis in the pathogenic oomycete Saprolegnia parasitica

Time: Mon 2019-12-16 10.00

Location: FB42, Roslagstullsbacken 21, AlbaNova, Stockholm (English)

Subject area: Biotechnology

Doctoral student: Elzbieta Rzeszutek , Glykovetenskap

Opponent: Professor Carol Munro, University of Aberdeen, Institute of Medical Sciences

Supervisor: Professor Vincent Bulone, Glykovetenskap

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The oomycete Saprolegnia parasitica is a fungus-like microorganism responsible for the fish disease saprolegniosis, which leads to important economic losses in aquaculture. Currently, there is no efficient method to control the infection and therefore methods for disease management are urgently needed. One of the promising approaches to tackle the pathogen is the inhibition of cell wall biosynthesis, specifically the enzymes involved in carbohydrate biosynthesis. The cell wall of S. parasitica consists mainly of cellulose, β-1,3 and β-1,6-glucans, whereas chitin is present in minute amounts only. The available genome sequence allowed the identification of six putative chitin (Chs) and cellulose (CesA) synthase genes. The main objective of this work was to characterize CHSs and CesAs from S. parasitica and test the effect of cell wall related inhibitors on pathogen growth. The tested inhibitors included nikkomycin Z, a competitive inhibitor of CHS as well as inhibitors of cellulose biosynthesis, namely flupoxam, CGA325'615 and compound I (CI). All drugs strongly reduced the growth of S. parasitica and inhibited the in vitro formation of chitin or cellulose, demonstrated by the use of a radiometric assay. The chemicals also affected the expression of some of the corresponding Chs and CesA genes.

One of the CHSs, namely SpCHS5, was successfully expressed in yeast and purified to homogeneity as a full length protein. The recombinant enzyme was biochemically characterized and demonstrated to form chitin crystallites in vitro. Moreover, our data indicate that SpCHS5 most likely occurs as a homodimer which can further assemble into larger multi-subunit complexes. Point mutations of conserved amino acids allowed us to identify the essential residues for activity and processivity of the enzyme.

In addition to the cell wall related inhibitors, a biosurfactant naturally produced by Pseudomonas species, massetolide A, was tested, showing strong inhibition of S. parasitica growth.

Altogether, our data provide key information on the fundamental mechanisms of chitin and cellulose biosynthesis in oomycetes and the biochemical properties of the enzymes involved. They also demonstrate that the enzymes involved in cell wall biosynthesis represent promising targets for anti-oomycete drugs, even when the corresponding polysaccharides, such as chitin, occur in small amounts in the cell wall.