Genome, transcriptome, and secretome analysis of wood decay fungus Postia placenta supports unique mechanisms of lignocellulose conversion

Los Alamos National Laboratory/Joint Genome Institute, P.O. Box 1663, Los Alamos, NM 87545, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.81). 02/2009; 106(6):1954-1959. DOI: 10.1073/pnas.0809575106

ABSTRACT Brown-rot fungi such as Postia placenta are common inhabitants of forest ecosystems and are also largely responsible for the destructive decay of wooden structures.
Rapid depolymerization of cellulose is a distinguishing feature of brown-rot, but the biochemical mechanisms and underlying
genetics are poorly understood. Systematic examination of the P. placenta genome, transcriptome, and secretome revealed unique extracellular enzyme systems, including an unusual repertoire of extracellular
glycoside hydrolases. Genes encoding exocellobiohydrolases and cellulose-binding domains, typical of cellulolytic microbes,
are absent in this efficient cellulose-degrading fungus. When P. placenta was grown in medium containing cellulose as sole carbon source, transcripts corresponding to many hemicellulases and to a
single putative β-1–4 endoglucanase were expressed at high levels relative to glucose-grown cultures. These transcript profiles
were confirmed by direct identification of peptides by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Also up-regulated
during growth on cellulose medium were putative iron reductases, quinone reductase, and structurally divergent oxidases potentially
involved in extracellular generation of Fe(II) and H2O2. These observations are consistent with a biodegradative role for Fenton chemistry in which Fe(II) and H2O2 react to form hydroxyl radicals, highly reactive oxidants capable of depolymerizing cellulose. The P. placenta genome resources provide unparalleled opportunities for investigating such unusual mechanisms of cellulose conversion. More
broadly, the genome offers insight into the diversification of lignocellulose degrading mechanisms in fungi. Comparisons with
the closely related white-rot fungus Phanerochaete chrysosporium support an evolutionary shift from white-rot to brown-rot during which the capacity for efficient depolymerization of lignin
was lost.

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