FIG 6.

A model of the induction (A), inhibition (B), and relief of inhibition (C) of cellulase production in N. crassa. After the degradation of cellulose and glucomannan by cellulases (endo- and exo-acting glucanases [orange]) and mannanase (blue), respectively, (gluco)mannodextrins outcompete cellodextrins extracellularly at the level of transport by the MFS-type transporter CDT-1. Intracellularly, cellodextrins and (gluco)mannodextrins are further cleaved into the corresponding glucose and mannose monomers by the action of the intracellular β-glucosidase (GH1-1) and β-mannosidase (GH2-1), respectively. (A) In the case of an intracellular balance between cello- and mannodextrins, an unknown signaling cascade will lead to the activation of the upstream transcription factor CLR-1, which induces expression of the downstream transcription factor CLR-2, which then evokes the major cellulolytic and mannanolytic responses. (B) In the Δgh2-1 deletion strain, undigested (gluco)mannodextrins accumulate in the cytosol, disrupting the intracellular balance of signaling molecules and outcompeting the positively inducing cellodextrins, in a way that the fungus is unable to determine the “adequate” amount of cellulase enzymes to be produced, eventually causing a reduced cellulase production. (C) When gh1-1 is deleted in the Δgh2-1 background (Δgh2-1 Δgh1-1 strain), the accumulating mannodextrins can be counterbalanced by the larger amount of undigested cellodextrins present in the cytosol, which reinforce the induction of the cellulolytic response and relieve the inhibition.