Originally published as Genetics Published Articles Ahead of Print on February 3, 2008.

Genetics, Vol. 178, 873-881, February 2008, Copyright © 2008
doi:10.1534/genetics.107.073148

A Novel Screening Method for Cell Wall Mutants in Aspergillus niger Identifies UDP-Galactopyranose Mutase as an Important Protein in Fungal Cell Wall Biosynthesis

Robbert A. Damveld^*,1, Angelique Franken^*, Mark Arentshorst^*, Peter J. Punt, Frans M. Klis, Cees A. M. J. J. van den Hondel^* and Arthur F. J. Ram^*,2

^* Institute of Biology Leiden, Leiden University, Molecular Microbiology, 2333 AL, Leiden, The Netherlands, TNO Quality of Life, Department of Microbiology, 3704 HE, Zeist, The Netherlands and Swammerdam Institute for Life Sciences, University of Amsterdam, BioCentrum Amsterdam, 1018 WV, Amsterdam, The Netherlands

² Corresponding author: Institute of Biology, Clusius Laboratory, Molecular Microbiology, Leiden University, Wassenaarseweg 64, 2333 AL, Leiden, The Netherlands.
E-mail: a.f.j.ram{at}biology.leidenuniv.nl

To identify cell wall biosynthetic genes in filamentous fungi and thus potential targets for the discovery of new antifungals, we developed a novel screening method for cell wall mutants. It is based on our earlier observation that the Aspergillus niger agsA gene, which encodes a putative -glucan synthase, is strongly induced in response to cell wall stress. By placing the agsA promoter region in front of a selectable marker, the acetamidase (amdS) gene of A. nidulans, we reasoned that cell wall mutants with a constitutively active cell wall stress response pathway could be identified by selecting mutants for growth on acetamide as the sole nitrogen source. For the genetic screen, a strain was constructed that contained two reporter genes controlled by the same promoter: the metabolic reporter gene PagsA-amdS and PagsA-H2B-GFP, which encodes a GFP-tagged nuclear protein. The primary screen yielded 161 mutants that were subjected to various cell wall-related secondary screens. Four calcofluor white-hypersensitive, osmotic-remediable thermosensitive mutants were selected for complementation analysis. Three mutants were complemented by the same gene, which encoded a protein with high sequence identity with eukaryotic UDP-galactopyranose mutases (UgmA). Our results indicate that galactofuranose formation is important for fungal cell wall biosynthesis and represents an attractive target for the development of antifungals.

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