Lignocellulose-degrading actinomycetes

Abstract

Increasing interest in the exploitation of plant biomass as a renewable resource has provided an impetus for research on microbial degradation of lignocellulose. This has traditionally been concentrated on the fungi, but lignocellulose-degrading prokaryotes are beginning to receive more attention. Strain improvement by genetic manipulation, and large-scale cultivation are more easily achieved in prokaryotes, and the actinomycetes are no exception. In addition, their growth as branching hyphae is well adapted to the penetration and degradation of insoluble substrates such as lignocellulose. Actinomycetes form an important part of the microbial community responsible for nutrient recycling in natural substrates. Their specific contribution to lignocellulose degradation in the environment has received only limited attention but their importance can be inferred from the ubiquity and diversity of species in which activity against lignocellulose has been demonstrated. Characterisation of the cellulolytic activity of actinomycetes has understandably received most attention, since cellulose is the major component of plant biomass and a potentially utilisable source of glucose. Actinomycete cellulases are inducible extracellular enzymes which appear to attack cellulose in much the same way as fungal hydrolytic cellulases. Actinomycete xylanases similarly conform to the basic patterns of production and activity established in other bacteria and fungi, but they have been relatively little-studied. Evidence for activity against lignin has only recently been obtained for actinomycetes, with the development of sensitive radiometric assays which permit detection of limited ligninolytic activity. There is little doubt that actinomycetes compare poorly with the whiterot fungi in relation to the extent of delignification achieved, particularly with wood lignocelluloses. Nevertheless, their ability to solubilise grass lignins may have a role in humification and biotechnological applications of lignocellulose conversion. The biochemistry of lignin degradation by actinomycetes remains poorly understood and the enzymes involved have yet to be identified. There is more information on the enzymology of cellulose and hemicellulose degradation, but how these different groups of enzymes and their component proteins interact to solubilise lignocellulose is largely unknown. Elucidation of the biochemistry of lignocellulose degradation and development of actinomycete-based systems for lignocellulose conversion should be greatly assisted by the application of the recombinant DNA techniques recently developed for this group of organisms.