CARBON CATABOLITE REPRESSION OF XYLANASE-I (XYN1) GENE-EXPRESSION IN TRICHODERMA-REESEI

The filamentous fungus Trichoderma reesei forms two specific, xylan-in ducible xylanases encoded by xyn1 and xyn2 to degrade the beta-1,4-D-x ylan backbone of hemicelluloses. This enzyme system is formed in the p resence of xylan, but not glucose. The molecular basis of the absence of xylanase I formation on glucose was the purpose of this study. Nort hern blotting of the xyn1 transcript as well as the use of the Escheri chia coil hygromycin B phosphotransferase-encoding gene (hph) as a rep orter consistently showed that the basal expression of xyn1 was affect ed by glucose, whereas its induction by xylan remained uninfluenced. T he repression of basal xyn1 transcription is mediated by the carbon ca tabolite repressor protein Cre1, which in vivo binds to two of four co nsensus sites (5'-SYG-GRG-3') in the xyn1 promoter, which occurred in the form of an inverted repeat. T. reesei strains, bearing a xyn1::hph reporter construct, in which four nucleotides from the middle of the inverted repeat had been removed, expressed hph on glucose at a level comparable to that observed during growth on a carbon catabolite derep ressing carbon source. Northern analysis of xyn1 expression in a T. re esei mutant strain (RUT C-30), which contains a truncated, non-functio nal cre1 gene, also confirmed basal transcription of xyn1. In this str ain, xyn1 transcription was still inducible by xylose or xylan to an e ven higher degree than in the wild-type strain, suggesting that induct ion overcomes glucose repression at the level of xyn1 expression, Base d on these data, we postulate that basal transcription of xyn1 is repr essed by glucose and mediated by an inverted repeat of the consensus m otif for Cre1-mediated carbon catabolite repression.