Cellodextrin Transporters of Neurospora crassa and their Utility in Saccharomyces cerevisiae During a Biofuel Production Process

The filamentous fungus, Neurospora crassa, is capable of depolymerizing and metabolizing plant cell walls. When grown on plant cell walls or pure cellulose, N. crassa upregulates an overlapping set of 114 genes. Amongst this set are 10 major facilitator superfamily transporters. I have shown that two of these, CDT-1 and CDT-2, transport cellodextrins, which are the major degradative product of fungal cellulases. Deletion of cdt-2 affects the growth of N. crassa on crystalline cellulose. Furthermore, diverse fungi transcriptionally upregulate orthologs of cdt-1 and cdt-2 when in contact with plant cell walls, suggesting that cellodextrin transporters are important to fungal interactions with plants. Engineering the cellodextrin transport pathway into Saccharomyces cerevisiae allows this yeast to ferment cellodextrins to ethanol with high yields, and facilitates the simultaneous saccharification and fermentation of cellulose to ethanol. Cellodextrin transport can be coupled to downstream hydrolysis or phosphorolysis of cellodextrins by a cellodextrin hydrolase or cellobiose phosphorylase, respectively. Cellodextrin transport circumvents a major limitation of yeast in fuel production: the inability to simultaneously transport and ferment pentose sugars and glucose to ethanol. S. cerevisiae did not evolve to co-ferment cellobiose and xylose and unintended consequences are likely. For example, we speculate that S. cerevisiae may not sense cellobiose as a fermentable carbon source.