Small chemical chromatin effectors alter secondary metabolite production in Aspergillus clavatus

The filamentous fungus Aspergillus clavatus is known to produce a variety of secondary metabolites (SM) such as patulin, pseurotin A, and cytochalasin E. In fungi, the production of most SM is strongly influenced by environmental factors and nutrients. Furthermore, it has been shown that the regulation of SM gene clusters is largely based on modulation of a chromatin structure. Communication between fungi and bacteria also triggers chromatin-based induction of silent SM gene clusters. Consequently, chemical chromatin effectors known to inhibit histone deacetylases (HDACs) and DNA-methyltransferases (DNMTs) influence the SM profile of several fungi. In this study, we tested the effect of five different chemicals, which are known to affect chromatin structure, on SM production in A. clavatus using two growth media with a different organic nitrogen source. We found that production of patulin was completely inhibited and cytochalasin E levels strongly reduced, whereas growing A. clavatus in media containing soya-derived peptone led to substantially higher pseurotin A levels. The HDAC inhibitors valproic acid, trichostatin A and butyrate, as well as the DNMT inhibitor 5-azacytidine (AZA) and N-acetyl-D-glucosamine, which was used as a proxy for bacterial fungal co-cultivation, had profound influence on SM accumulation and transcription of the corresponding biosynthetic genes. However, the repressing effect of the soya-based nitrogen source on patulin production could not be bypassed by any of the small chemical chromatin effectors. Interestingly, AZA influenced some SM cluster genes and SM production although no Aspergillus species has yet been shown to carry detectable DNA methylation.