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Database mining and transcriptional analysis of genes encoding inulin-modifying enzymes of Aspergillus niger

Xiao-Lian Yuan^1,

Coenie Goosen^2,3,

¹ Institute of Biology Leiden, Leiden University, Fungal Genetics Research Group, Wassenaarseweg 64, 2333 AL Leiden, The Netherlands
² Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute (GBB), University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands
³ Centre for Carbohydrate Bioprocessing TNO-University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands
⁴ Microbiology, Fungal Genomics Group, Wageningen University, Dreijenlaan 2, 6703 HA Wageningen, The Netherlands
⁵ TNO Quality of Life, Business Unit Innovative Ingredients and Products, Rouaanstraat 27, 9723 CC Groningen, The Netherlands
⁶ TNO Quality of Life, Business Unit Microbiology, Utrechtseweg 48, 3500 AJ Zeist, The Netherlands

Correspondence
Arthur F. J. Ram
ram{at}rulbim.leidenuniv.nl

As a soil fungus, Aspergillus niger can metabolize a wide variety of carbon sources, employing sets of enzymes able to degrade plant-derived polysaccharides. In this study the genome sequence of A. niger strain CBS 513.88 was surveyed, to analyse the gene/enzyme network involved in utilization of the plant storage polymer inulin, and of sucrose, the substrate for inulin synthesis in plants. In addition to three known activities, encoded by the genes suc1 (invertase activity; designated sucA), inuE (exo-inulinase activity) and inuA/inuB (endo-inulinase activity), two new putative invertase-like proteins were identified. These two putative proteins lack N-terminal signal sequences and therefore are expected to be intracellular enzymes. One of these two genes, designated sucB, is expressed at a low level, and its expression is up-regulated when A. niger is grown on sucrose- or inulin-containing media. Transcriptional analysis of the genes encoding the sucrose- (sucA) and inulin-hydrolysing enzymes (inuA and inuE) indicated that they are similarly regulated and all strongly induced on sucrose and inulin. Analysis of a creA mutant strain of A. niger revealed that expression of the extracellular inulinolytic enzymes is under control of the catabolite repressor CreA. Expression of the inulinolytic enzymes was not induced by fructose, not even in the creA background, indicating that fructose did not act as an inducer. Evidence is provided that sucrose, or a sucrose-derived intermediate, but not fructose, acts as an inducer for the expression of inulinolytic genes in A. niger.

A supplementary table and figure are available with the online version of this paper.

These authors contributed equally to this work.

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