An apparently novel 1,3-β-glucan synthase from the oomycete Saprolegnia monoica has been characterized. The enzyme exhibits properties that differ markedly from those of the enzyme previously described [Fèvre, M. & Dumas, C. (1977). J Gen Microbiol 103, 297-306] as it is active at alkaline pH, stimulated by the divalent cations Ca2+, Mg2+ and Mn2+, and appears to be located mainly in the apical part of the hypha. Taking into consideration the differences in pH optimum and effect of divalent ions, each enzyme activity could be assayed in the presence of the other. The insoluble polymeric product of the enzyme with alkaline pH optimum was characterized as a linear 1,3-β-glucan. Comparisons of the general properties of 1,3-β-glucan synthases suggest that enzymes from the oomycetes are more closely related to enzymes from higher plants than to those of true fungi, reflecting the fact that the oomycetes are highly divergent from chitinous fungi.
Keywords: 1,3-β-glucan synthase; Saprolegnia monoica; apical growth; cation dependency; cell wall.