The transcriptional activator XlnR regulates both xylanolytic and endoglucanase gene expression in Aspergillus niger

Abstract

The expression of genes encoding enzymes involved in xylan degradation and two endoglucanases involved in cellulose degradation was studied at the mRNA level in the filamentous fungus Aspergillus niger. A strain with a loss-of-function mutation in the xlnR gene encoding the transcriptional activator XlnR and a strain with multiple copies of this gene were investigated in order to define which genes are controlled by XlnR. The data presented in this paper show that the transcriptional activator XlnR regulates the transcription of the xlnB, xlnC, and xlnD genes encoding the main xylanolytic enzymes (endoxylanases B and C and beta-xylosidase, respectively). Also, the transcription of the genes encoding the accessory enzymes involved in xylan degradation, including alpha-glucuronidase A, acetylxylan esterase A, arabinoxylan arabinofuranohydrolase A, and feruloyl esterase A, was found to be controlled by XlnR. In addition, XlnR also activates transcription of two endoglucanase-encoding genes, eglA and eglB, indicating that transcriptional regulation by XlnR goes beyond the genes encoding xylanolytic enzymes and includes regulation of two endoglucanase-encoding genes.

FIG. 1

Northern blot analysis of expression of A. niger genes encoding cellulose- and xylan-degrading enzymes. (A) Time course of induction in A. niger NW205::130 (wt) and NXA1-4 (xlnR1) (a loss-of-function mutant). Both strains were cultured for 18 h in medium containing 3% d-fructose (Fr), and mycelia were subsequently transferred to medium containing 1% xylan (Xa) or 1% d-xylose (Xo) and incubated for the times indicated. Each lane contained 10 μg of total RNA, which was checked by hybridization with the 18S rRNA probe. Blots were hybridized with gene-specific probes as indicated. (B) Comparison of expression of genes encoding cellulose- and xylan-degrading enzymes in A. niger N902 (wt), NXA1-4 (xlnR1), and N902-pIM230-25.12 (XlnR⁺) (N902 with multiple copies of xlnR) upon transfer to medium containing 1% d-xylose for 6 h after growth for 18 h in medium containing 1% d-fructose. A Northern blot analysis was performed exactly as described above. The signal intensities of the different blots cannot be compared to each other due to the unknown specific activities of the probes used and the different exposure times used for the various blots.