Wood-derived biomass offers a promising source for cellulose-based fuels, but efforts to exploit this energy have been thwarted by the need for methods to deal with a component of the plant cell wall that binds cellulose and interferes with enzymatic processing.
Termites have developed a natural workaround for this problem. Over the course of evolution, the various ‘lower termite’ species have formed an essential partnership with bacteria and protists dwelling within their gut, these derive support from their termite hosts and in turn facilitate the digestion of the insects’ woody diet.
Glycosyl hydrolase family (GHF) enzymes produced by these symbionts are a key component in the cellulose digestion process, enabling efficient cellulose processing without the need for lignin breakdown. “Some of the enzymes that we have found have more than 10-fold higher activity than current industrial enzymes,” says Shigeharu Moriya of the RIKEN Advanced Science Institute in Wako. Since 2001, Moriya and colleagues have been working to characterize these enzymes, and they have now published their analysis of the various GHFs expressed within the gut protist communities of four lower termite species as well as a related wood-eating cockroach1.
These protists are exceptionally challenging to culture and analyze individually, but can be characterized collectively via ‘metagenomic’ techniques that make it possible to assemble massive gene catalogues from a diverse mixture of cell types. This approach revealed a total of 154 clones representing variants of five different GHFs, and the researchers used this sequence data to assemble a phylogenetic tree—essentially a detailed timeline of the evolutionary history of these genes.
GHF5 and 7 were represented in every termite symbiont community investigated, suggesting that their evolution either precedes or coincides with the emergence of termite–protist symbiosis. Interestingly, the data suggest that GHF5 may have been initially acquired by protists from bacteria over the course of one or more ancient gene transfer events. GHF7, on the other hand, appears to have evolved specifically within protists.
The other three enzyme classes—GHF10, 11 and 45—are less broadly conserved, and the author speculate that they provide support for the core GHF5–GHF7 cellulose degradation machinery. “This system is well conserved among various termites, and it may be composed of high-performance enzymes,” says Moriya. His team is now partnering with other RIKEN teams to develop novel techniques for characterizing the metabolic pathways of these protist communities in an effort to identify additional factors that expedite biomass processing.
The corresponding author for this highlight is based at the Biosphere Oriented Biology Research Unit, RIKEN Advanced Science Institute