Induction of lignocellulose degrading enzymes in Neurospora crassa by cellodextrins

Neurospora crassa colonizes burnt grasslands in the wild and metabolizes both cellulose and hemicellulose from plant cell walls. When switched from a favored carbon source such as sucrose to cellulose, N. crassa dramatically upregulates expression and secretion of a wide variety of genes encoding lignocellulolytic enzymes. However, the means by which N. crassa and other filamentous fungi sense the presence of cellulose in the environment remains unclear. Here, I show that a N. crassa mutant carrying deletions of two genes encoding predicted extracellular β-glucosidase enzymes and one intracellular β-glucosidase enzyme (d3βG) lacks β‐glucosidase activity, but efficiently induces cellulase gene expression and cellulolytic activity in the presence of cellobiose as the sole carbon source. These data indicate that cellobiose, or a modified version of cellobiose, functions as an inducer of lignocellulolytic gene expression and activity in N. crassa. In addition, I have identified two cellodextrin transporters involved in sensing cellulose. A N. crassa mutant carrying deletions for both transporters is unable to induce cellulase gene expression in response to crystalline cellulose. Furthermore, a mutant lacking β- glucosidase enzymes and transporters (d3βGdT) does not induce cellulase gene expression in response to cellobiose