Little is known about the mechanism of pathway-specific induction of extracellular enzyme systems in fungi. Induction of polysaccharide degrading enzyme systems, as e.g.xylanases, depends on low molecular weight inducers, which can be taken up by the organism. In Aspergilli, has been shown that in addition to xylobiose, D-xylose is able to induce the xylanolytic system. However, it is not clear whether the induction is the direct effect of D-xylose or that the inducing compound is the result of a transglycosylation reaction, e.g catalysed by b-xylosidase. The gene encoding A. niger b-xylosidase, xlnD, has been cloned and the role of b-xylosidase in the induction process studied. Furthermore, A. niger mutants with decreased xylanolytic gene expression were isolated by using a xylan induction-responsive element of the endo-xylanase encoding gene xInA of A. tubingensis. Endo-xylanase activity of these mutants decreased a 300-S00-fold in comparison to the wild-type strain. Also a strong decrease is found for b-xylosidase activity in these mutants. By mutant complementing, the transcriptional activator xlnR was isolated. XlnR encodes a protein of 875 amino acids with a domain capable to form a Zn binuclear cluster. Besides this region no extensive homology was found to other transcription activators. By sequencing the xlnR allele of three loss-of-function mutants, a mutation was found in xlnR in all cases, excluding that the isolated gene is a suppressor. Using in vitro binding assays and footprinting techniques, the target sites in the promoter of xlnA have been determined to be 5'GGCTAA-3. Activation of transcription by XlnR is not limited to genes involved in xylan degradation, also genes encoding endoglucanases are activated.
Fungal Genet. Newsl. 46 (Supl):
Full conference title:
Fungal Genetics Conference 20th
- Fungal Genetics Conference 20th (1999)