Towards elucidation of the lignin degradation pathway in actinomycetes

Six biodegradative actinomycete strains were grown on a dimeric model lignin compound of the β-aryl ether type. Although only two strains, Thermomonospora mesophila and Streptomyces badius, utilized the compound as a carbon and energy source and produced substantial amounts of monomeric products, all of the strains could demethylate the substrate and oxidize Cα on the phenylpropane side-chain. Streptomyces sp. EC1 produced small amounts of aromatic acids and unidentified lignin-derived products when grown on straw. This organism also produced cell-bound demethylase requiring H2O2 and Mn2+, protocatechuate 3,4-dioxygenase and β-carboxymuconate decarboxylase activity in response to growth on low-molecular-mass aromatic compounds but not lignocellulose or its polysaccharide components. Extracellular peroxidase and catalase activity were detected in all of the strains. These data are used to propose a scheme by which actinomycete attack of the lignin component of plant biomass can be envisaged.