Production and properties of xylanases from thermophilic actinomycetes

30 strains of xylanolytic thermophilic actinomycetes were isolated from composted grass and cattle manure and identified as members of the genera Thermomonospora, Saccharomonospora, Microbispora, Streptomyces and Actinomadura. Screening of these strains for extracellular xylanase indicated that strains of Saccharomonospora and Microbispora generally were poor xylanase producers (0.5-1.5 U/ml) whereas relatively high activities were observed in cultures of Streptomyces and Actinomadura (4-12 U/ml). A preliminary characterization of the enzymes of strains of the latter genera suggested that xylanases of all the strains of Actinomadura exhibited higher thermostabilities than those of Streptomyces. To evaluate the potential of thermophilic Actinomadura for industrial applications, xylanases of three strains were studied in more detail. The highest activity levels for xylanases were observed in cultures grown on xylan and wheat bran. The optimal pH and temperature for xylanase activities ranged from 6.0 to 7.0 and 70 to 80 degrees C. The enzymes exhibited considerable thermostability at their optimum temperature. The half-lives at 75 degrees C were in the range from 6.5 to 17 h. Hydrolysis of xylan by extracellular xylanases yielded xylobiose, xylose and arabinose as principal products. Estimated by the amount of reducing sugars liberated the degree of hydrolysis was 55 to 65%. Complete utilization of xylan is presumably achieved by beta-xylosidase activities which could be shown to be largely cell-associated in the 3 Actinomadura strains.