β-1-3- and β-1-4-glucan synthesis by membrane fractions from the fungus Saprolegnia.

The β-glucan synthetase activity of the fungus Saprolegnia monoica was assayed by supplying UDP-glucose to membrane fractions of mycelial homogenate. The analysis of glucan products by hydrolysis with various β-glucanases and by chromatography show that both β-1-3- and β-1-4-linkages are formed at high substrate concentrations. In the absence of MgCl2, β-1-3-linked glucans are mainly produced. By increasing MgCl2 concentrations the total synthesis activity and β-1-3-linkages production are reduced. At low substrate concentrations in the presence of MgCl2, β-1-4-linked glucans are the only polysaccharide synthesized. Electron microscopy of radioactive products, synthesized by original membrane fractions or by membrane fractions isolated from continuous sucrose density gradients, shows microfibrils when the assays are conducted at high substrate concentrations in the absence of MgCl2.