The Aspergillus niger MADS-box transcription factor RlmA is required for cell wall reinforcement in response to cell wall stress

In Aspergillus niger, the genes coding for glutamine:fructose-6-phosphate amidotransferase (gfaA) and alpha-1,3-glucan synthase (agsA) are induced in response to cell wall stress. In silico analysis of the promoter region of the two genes revealed the presence of putative DNA binding sites for transcription factors involved in stress responses, including sites identical to the Saccharomyces cerevisiae Rlm1p and Msn2p/Msn4p transcription factors. Promoter analysis indicated that the induction of the agsA gene in response to cell wall stress is fully dependent on a putative Rlm1p binding site in its promoter region. Database searches revealed the presence of S. cerevisiae Rlm1p homologues in most filamentous fungi examined, including A. niger. Deletion of the RLM1 homologue, named rlmA in A. niger, completely eliminated the induction of agsA and resulted in a twofold reduced induction of gfaA during Calcofluor White-induced cell wall stress. The rise in cell wall chitin in the presence of Calcofluor White was also affected in the rlmA deletion strain. In addition, the deletion strain was more sensitive towards cell wall stress agents. Our results indicate that A. niger responds to cell wall stress by transcriptional activation of cell wall reinforcing genes including agsA and gfaA through an Rlm1p-like transcription factor. We propose that such a cell wall salvage mechanism is wide spread in filamentous fungi.