Lignin-modifying enzymes of the white rot basidiomycete Ganoderma lucidum.

Lignin, the second most abundant renewable organic polymer on earth, is a major component of wood. Because of the importance of wood and other lignocellulosics as a renewable resource for the production of paper products, feeds, chemicals, and fuels, there has been an increasing research emphasis on the fungal degradation of lignin (5, 30). White rot fungi are believed to be the most effective lignin-degrading microbes in nature. A majority of the previous studies have focused on the lignin-degrading enzymes of Phanerochaete chrysosporium and Trametes versicolor (23, 44). Recently, however, there has been a growing interest in studying the lignin-modifying enzymes of a wider array of white rot fungi, not only from the standpoint of comparative biology but also with the expectation of finding better lignin-degrading systems for use in various biotechnological applications (14, 24, 36, 39). Three major families of fungal lignin-modifying enzymes (LMEs) are laccases, manganese-dependent peroxidases (MnPs), and lignin peroxidases (LiPs) (5, 24, 30, 50). These LMEs can oxidize phenolic compounds thereby creating phenoxy radicals, while nonphenolic compounds are oxidized via cation radicals (5, 25, 30). LiP and MnP oxidize nonphenolic aromatic compounds with high oxidation-reduction potentials (21), the major components of the lignin polymer. Laccase oxidizes nonphenolic aromatic compounds with relatively low oxidation-reduction potentials (29, 56). In the presence of low-molecular-weight mediators, laccases can also oxidize nonphenolic substrates with high oxidation-reduction potentials (6, 9, 16) as well as certain xenobiotics (27). Preliminary studies in our laboratory showed the presence of lip gene-homologous sequences in the genomic DNA of several genera of wood rot fungi (13). Subsequent screening on plates containing the polymeric dye poly R-478, decolorization of which is correlated with lignin degradation (22), led to the selection of a strain of Ganoderma lucidum for further studies, based on its rapid growth rate and extensive decolorization of poly R-478 on solid media. G. lucidum is one of the most important and widely distributed white rot fungi in North America and is associated with the degradation of a wide variety of hardwoods (1). Previous studies of G. lucidum have mainly concentrated on the medicinal properties of this fungus (reviewed in reference 28) and, except for two brief preliminary reports (26, 41), little is known about the ligninolytic system of this organism. In this report, we describe the production of LMEs by G. lucidum under different culturing conditions, with emphasis on its laccase.