XYLANASES FROM THE FUNGAL MAIZE PATHOGEN COCHLIOBOLUS-CARBONUM

The fungal pathogen Cochliobolus carbonum secreted xylanase into the culture medium when grown on xylan or isolated maize cell walls. When the fungus was grown on maize cell walls, sucrose at 0·2% stimulated xylanase production but at 0·8% or above completely suppressed production. The activity could be assayed by either reduction in viscosity of a xylan solution or by the release of reducing sugars from xylan. One major and two minor xylanase enzymes were resolved by cation exchange high pressure liquid chromatography. The major enzyme, xylanase I, was purified 28-fold to homogeneity by ion exchange and hydrophobic interaction chromatography. The three xylanases had Mrs by sodium dodecyl sulphate gel electrophoresis of 22000 to 24000 but by gel filtration behaved as molecules of Mr 8500. All three xylanases had pIs of greater than 9·3 and cleaved xylan in an endo manner. While all three were optimally active at pH 5·0. they differed in their activity above pH 5·5. The activity of xylanase I was fully maintained between pH 4·0 and 8·0. However, the activity of the minor xylanase II and to a lesser extent the minor xylanase III dropped at pH 6·0 and above. Xylanase I was digested with trypsin and the resulting peptides separated and sequenced. One peptide of nine residues showed 89% identity to four bacterial endo-xylanases. © 1992.