Sacchariiication of Straw by Actinomycete Enzymes

Over 200 strains of actinomycetes, representing nine distinct genera, were screened directly for the ability to release reducing sugar from ball-milled wheat straw, using a microtitre plate assay system. Xylanase activity was detected in nearly all of the strains examined while activities against purified cellulosic substrates were less widespread and relatively low. Straw saccharification resulted from cooperative enzyme action and sugar yields were not simply correlated with substrate particle size. Straw-saccharifying activity was further characterized in selected strains comprising five representatives of the genera Thermomonospora and Streptomyces, one Micromonospora strain and the type strain of Microbispora bispora. Common features included optimal saccharification of straw in the pH range 64-9.0 and xylose and its oligomers as the principal products, although low concentrations of glucose were also detected. Optimal activity and increased stability at 70 "C was a feature of enzyme preparations from Thermomonospora and thermophilic Streptomyces strains. P-Xylosidase and P-glucosidase activities were largely intracellular, but significant amounts of extracellular P-xylosidase activity were also found in two strains. Other enzymes involved in straw saccharification include acetylesterase and arabinofuranosidase, and these activities were detected in all strains. Acetylesterase and arabinofuranosidase activities were largely extracellular, but in some strains significant amounts of intracellular activity were also detected. Agricultural residues such as grass lignocelluloses represent large renewable resources for which enzymic generation of fermentable sugars is one of a number of alternative strategies currently under investigation. Wheat straw is a widely available substrate and its disposal also presents an environmental problem. The suitability of wheat straw for bioconversion processes is further enhanced by its relatively low lignin content (< 20%, w/w) and high content (up to 30 %, w/w) of readily degradable hemicellulose (arabinoxylan) (Ladisch et al., 1983). Actinomycetes are widely distributed in natural environments, such as soils and composts, where they make an important contribution to nutrient recycling and humification. They are therefore a potentially useful source of plant biomass-degrading enzymes, and activity against the major components (lignin, hemicellulose and cellulose) has been identified in many strains (McCarthy, 1987). However, the enzymology of lignocellulose degradation by actinomycetes remains poorly understood. It is likely that a concerted attack against all components of the substrate, probably involving synergistic action between enzymes, takes place as has been described for fungal cellulases (Wood & McCrae, 1978) and hemicellulases (Biely et al., 1986). Actinomycete cellulases and xylanases are inducible extracellular enzymes which are often produced simultaneously (Kluepfel & Ishaque, …