Figure 6.

Model for the regulation of GA-induced gene expression in A. niger. (A) GA-induced gene expression, with pgaX as an example, is controlled via interaction of the transcriptional activator (GaaR) and transcriptional repressor (GaaX) in combination with CreA-mediated carbon catabolite repression. (A) In the presence of fructose (a nonrepressible, noninducing carbon source) pgaX expression is prevented because GaaX inhibits GaaR activation. The question mark indicates that the mechanism by which GaaX controls GaaR activity is unknown. (B) In the presence of GA, GA itself or a derivative of GA is predicted to bind to GaaX. The binding of the inducer to GaaX is expected to activate GaaR. GaaX is induced and remains cytosolic but the presence of the inducer keeps GaaX inactive. (C) In the ΔgaaX strain, GaaR is no longer kept inactive by GaaX and therefore is constitutively active, resulting in constitutive expression of pgaX. (D) In the ΔgaaX strain, the presence of glucose leads to CreA-mediated repression leading to reduced expression of pgaX and possibly other pectinolytic genes. (E) Deletion of both gaaX and creA results in constitutive expression of pgaX even in the presence of glucose. The yellow triangles represent putative CreA binding sites. The green/blue circle represents a putative GaaR-binding site. The red circle represents the postulated inducing sugar.