Genetic Analysis of Brown Rot Fungus Reveals Unique Enzyme Systems for Breaking Down Cellulose; Possible Application for More Efficient Cellulosic Biofuels Processes
|Scanning electron micrograph showing the thread-like fungus ramifying through wood cells. Photo: Tom Kuster (FPL). Click to enlarge.|
An international team led by scientists from the US Department of Energy (DOE) Joint Genome Institute (JGI) and the US Department of Agriculture Forest Service, Forest Products Laboratory (FPL) has analyzed the genome, transcriptome, and secretome of Postia placenta, a brown rot fungus, and found unique extracellular enzyme systems, including an unusual repertoire of extracellular glycoside hydrolases.
P. placenta rapidly deconstructs the cellulose in wood, but does so using different mechanisms than used by cellulolytic microbes; the genes encoding exocellobiohydrolases and cellulose-binding domains, which are typical of cellulolytic microbes, are absent in Postia. The research, conducted by more than 50 authors, is reported in the 4 February online edition of the Proceedings of the National Academy of Sciences (PNAS).
Among the challenges to more cost-effective production of biofuels from cellulosic biomass is to find effective means to work around the polymer lignin, the scaffolding that endows the plant’s architecture with rigidity and protection from pests. By doing so, the organic compound cellulose—the long chain of glucose (sugar) units can be unbound, broken down, fermented, and distilled into liquid transportation fuel.
Postia has, over its evolution, shed the conventional enzymatic machinery for attacking plant material. Instead, the evidence suggests that it utilizes an arsenal of small oxidizing agents that blast through plant cell walls to depolymerize the cellulose. This biological process opens a door to more effective, less-energy intensive and more environmentally-sound strategies for more lignocellulose deconstruction.—Dan Cullen, FPL scientist and one of the senior authors
Few organisms in nature can efficiently breakdown lignin into smaller, more manageable chemical units amenable to biofuels production. The exceptions are the basidiomycete fungi, which include white-rot and brown-rot—wood-decayers and essential caretakers of carbon in forest systems. In addition, brown-rot fungi have significant economic impact because their ability to wreak havoc with wooden structures. A significant portion of the US timber harvest is diverted toward replacing such decayed materials.
Unlike white-rot fungi, previously characterized by DOE JGI and FPL, which simultaneously degrades lignin and cellulose, brown-rot rapidly depolymerizes the cellulose in wood without removing the lignin. Up until this study, the underlying genetics and biochemical mechanisms were poorly understood.
Postia’s genome sequence was the first step in the process that the scientific team employed to home in on the subset of data, the transcriptome, that encodes the specific enzyme activity, and the secretome, the products exported from the cell.
For the first time we have been able to compare the genetic blue prints of brown-rot, white-rot and soft-rot fungi which play a major role in the carbon cycle of our planet. Such comparisons will increase our understanding of the diverse mechanisms and chemistries involved in lignocellulose degradation. This type of information may empower industrial biotechnologists to devise new strategies to enhance efficiencies and reduce costs associated with biomass conversion for renewable fuels and chemical intermediates.—Randy Berka, senior author and Director of Integrative Biology, at Novozymes, Inc.
Diego Martinez et al. (2009) Genome, transcriptome, and secretome analysis of wood decay fungus Postia placenta supports unique mechanisms of lignocellulose conversion. PNAS Early Edition doi: 10.1073/pnas.0809575106