MpkA-Dependent and -Independent Cell Wall Integrity Signaling in Aspergillus nidulans

Cell wall integrity signaling (CWIS) maintains cell wall biogenesis in fungi, but only a few transcription factors (TFs) and target genes downstream of the CWIS cascade in filamentous fungi are known. Because a mitogen-activated protein kinase (MpkA) is a key CWIS enzyme, the transcriptional regulation ofmpkA and of cell wall-related genes (CWGs) is important in cell wall biogenesis. We clonedAspergillus nidulans mpkA ;rlmA , a TF gene orthologous toSaccharomyces cerevisiae RLM1 that encodes Rlm1p, a major Mpk1p-dependent TF that regulates the transcription ofMPK1 besides that of CWGs; and Answi4 and Answi6 , homologous toS. cerevisiae SWI4 andSWI6 , encoding the Mpk1p-activating TF complex Swi4p-Swi6p, which regulates CWG transcription in a cell cycle-dependent manner.A. nidulans rlmA andmpkA cDNA functionally complementedS. cerevisiae rlm1 Δ andmpk1 Δ mutants, respectively, but Answi4 and Answi6 cDNA did not complementswi4 Δ andswi6 Δ mutants. We constructedA. nidulans rlmA , Answi4 and Answi6 , andmpkA disruptants (rlmA Δ, Answi4 Δ Answi6 Δ, andmpkA Δ strains) and analyzedmpkA and CWG transcripts after treatment with a β-1,3-glucan synthase inhibitor (micafungin) that could activate MpkA via CWIS. Levels ofmpkA transcripts in the mutants as well as those in the wild type were changed after micafungin treatment. The β-glucuronidase reporter gene controlled by thempkA promoter was expressed in the wild type but not in thempkA Δ strain. Thus,mpkA transcription seems to be autoregulated by CWIS via MpkA but not by RlmA or AnSwi4-AnSwi6. The transcription of most CWGs except α-1,3-glucan synthase genes (agsA andagsB ) was independent of RlmA and AnSwi4-AnSwi6 and seemed to be regulated by non-MpkA signaling. The transcriptional regulation ofmpkA and of CWGs via CWIS inA. nidulans differs significantly from that inS. cerevisiae .