Cellulotastic

A colony of the fungus Neurospora crassa (left) growing on cellulose and the yeast Saccharomyces cerevisiae. Sugar transporters from Neurospora that have been inserted into the yeast are tagged with green fluorescent protein. (Image: Jamie Cate and Susan Jenkins, UC Berkeley & EBI)

Reseachers at the University of California, Berkeley have been working on how and what yeast can digest. They have taken genes from a grass-eating fungi (Neurospora crassa) and put them into yeast. This new set of genes allows the yeast to produce alcohol from cellulose, a material that is normally indigestible. N. crassa is one of the rare yeasts that can digest cellulose because its preferred diet is fire-damaged plants. The genes were located by doing a genome-wide systems analysis to see which genes are turned on when the fungus grows on cellulose. The researchers were able to locate a family of genes which produces proteins that transport sugars into the Neurospora cell to be used a fuel and some transporters, in particular, that allow the cell to import a few types of cellodextrins (specifically cellobiose, cellotriose, and cellotetraose), simple-ish sugars.

Why should we care what yeast are able to digest? The biofuels industry has been struggling to make cellulosic ethanol economically feasible. Right now the industry is using brewers yeast, the single-celled fungus (Saccharomyces cerevisiae) to turn sugar or other simple carbohydrates into alcohol. So far cellulose has been a bit of a problem as it is a pretty tough molecule; it is composed of glucose (simple sugar) molecules all linked together into long chains. Its a molecule that you can find in abundance in various materials such as corn stalks, leaves and cobs (and the US uses a whole lotta corn!) as well as paper waste or any other plant material. Normally these long chains have to be broken up into smaller cellodextrins by enzymes called cellulases using a method called saccharification. Then another enzyme called beta-glucosidase is added to break down the cellodextrins into their simple sugar components. That's all before the yeast even get introduced to the system! If the N. crassa genes (cellodextrin transporters) can be inserted into the industrial strains of yeast that are currently being used for ethanol production the efficiency of the fermentation process can be greatly improved.

The paper appeared in this weeks ScienceExpress:
Galazka, Jonathan M., et al. (2010) Cellodextrin transport in yeast for improved biofuel production. Science: published online. (DOI: 10.1126/science.1192838)

Here is UC Berkeley's new article about the paper:
http://berkeley.edu/news/media/releases/2010/09/09_neurospora.shtml