Deletion of the col-26 Transcription Factor Gene and a Point Mutation in the exo-1 F-Box Protein Gene Confer Sorbose Resistance in Neurospora crassa.

L-Sorbose induces hyperbranching of hyphae, which results in colonial growth in Neurospora crassa. The sor-4 gene, which encodes a glucose sensor that acts in carbon catabolite repression (CCR), has been identified as a sorbose resistance gene. In this study, we found that the deletion mutant of col-26, which encodes an AmyR-like transcription factor that acts in CCR, displayed sorbose resistance. In contrast, the deletion mutants of other CCR genes, such as a hexokinase (hxk-2), an AMP-activated S/T protein kinase (prk-10), and a transcription factor (cre-1), showed no sorbose resistance. Double mutant analysis revealed that the deletion of hxk-2, prk-10, and cre-1 did not affect the sorbose resistance of the col-26 mutant. Genes for a glucoamylase (gla-1), an invertase (inv), and glucose transporters (glt-1 and hgt-1) were highly expressed in the cre-1 mutant, even in glucose-rich conditions, but this upregulation was suppressed in the Δcre-1;Δcol-26a double-deletion mutant. Furthermore, we found that a dgr-2(L1)a mutant with a single amino-acid substitution, S11L, in the F-box protein EXO-1 displayed sorbose resistance, unlike the deletion mutants of exo-1, suggesting that the function of EXO-1 is crucial for the resistance. Our data strongly suggest that CCR directly participates in sorbose resistance, and that COL-26 and EXO-1 play important roles in regulating the amylase and glucose transporter genes during CCR.