POLYGALACTURONASE ISOLATED FROM THE CULTURE OF THE PSYCHROPHILIC FUNGUS SCLEROTINIA-BOREALIS

A polygalacturonase was isolated from the culture medium of Sclerotini a borealis, a psychrophilic fungus that grows on lawn and wheat seedli ng under the snow in winter and induces the snow mold disease. Pectic acid was a better substrate of this enzyme than pectin when the activi ty was determined by measuring the reducing sugar produced. However, w hen the activity was measured by viscosity change, the viscosity of pe ctin decreased more rapidly than that of pectic acid. The results of v iscosity change apparently indicate that the polygalacturonase catalyz es pectin hydrolysis as an endo-type enzyme. Highly methyl-esterified pectin was a poor substrate, as determined by measurements of reducing sugar production and viscosity change. It is suggested from the resul ts that the methoxy group of pectin affects the polygalacturonase reac tion. A reaction mechanism was proposed for the polygalacturonase reac tion. Molecular mass of this enzyme was 40 kDa and its isoelectric poi nt was pH 7.5. Optimum pH of the enzyme reaction was 4.5 and its optim um temperature was 40-50 degrees C. Thirty percent of the maximum acti vity was observed at 5 degrees C, but it was only slightly active abov e 60 degrees C. The activity was preserved for more than 2 years at 5 degrees C and pH 4.5, but it was lost when kept at room temperature ov ernight or heated at 50 degrees C for 30 min. The amino acid sequence of the N-terminal region of the psychrophilic polygalacturonase of Scl erotinia borealis is compared with those of polygalacturonases of meso philic fungi. The function of this enzyme against the target plants is discussed with reference to the reaction of polygalacturonases of mes ophilic fungi.