Appl Microbiol Biotechnol (2012) 95:1389–1404 deposition of activating histone marks such as lysine acetyla- tion in H3 and H4. In other words, both enzymatic activities removing repressive heterochromatic marks and placing acti- vating euchromatic marks must be recruited to SM gene promoters under secondary metabolic conditions. Usually, HAT complexes are recruited to nucleosome-free regions in promoters of genes by specific DNA-binding proteins. This leads to a sequence of downstream events such as histone hyperacetylation, nucleosome remodeling, assembly of the basic transcriptional machinery, and subsequent RNA poly- merase II recruitment culminating in transcriptional activity of the promoter (Berger 2002; 2007). The chromatin code is different for different SM clusters Not for all studied SM clusters, a pathway activator performing the first step in recruitment is known and the broad-domain activator LaeA is not predicted to possess DNA-binding activity. For example, for the A. nidulans, pen- icillin or orsellinic acid/F9775 (ORS) (encoding for archetyp- ical polyketide orsellinic acid and F9775A, F9775B cathepsin K inhibitors) clusters no pathway-specific activators have been identified and activation is mediated by general tran- scription factors (Espeso et al. 1993; MacCabe et al. 1990; Sanchez et al. 2009; Schroeckh et al. 2009). Still, the genes are strongly induced by SM-activating conditions. Are different activators responsible for recruitment of putative H3K9me3 lysine demethylases (KDMs) to reverse heterochromatic marks and for attracting HATs to place activating marks? At the moment, there is only scattered information available from chromatin studies at SM clusters and to answer these crucial questions is not yet possible. Moreover, only the apparently most relevant chromatin modifications have been studied so far but a much larger array of additional modifications are possible and may play an essential role in SM gene silencing and activation (Kouzarides 2007). Histone acetylation—an essential general mark for SM gene activation In all eukaryotes, histones are acetylated at several positions during gene activation and several multi-subunit complexes such as Saga/Ada or NuA4 are known to possess histone acetyltransferase activities (Brownell and Allis 1995; Brownell et al. 1996; Grant et al. 1997; Grant et al. 1998). Whereas gene regulation by HATs was extensively studied in many organ- isms, comparatively little molecular information on the func- tion of these complexes is available in filamentous fungi. In N. crassa, Grimaldi and collegues showed that the light-response regulator WC1 recruits the catalytic subunit NGF-1 to light regulated genes and catalyses H3K14 acetylation during gene 1395 activation (Grimaldi et al. 2006). In A. nidulans, the catalytic and regulatory subunits of Saga/Ada, GcnE, and AdaB, re- spectively, are required for induction of conidiation ((Reyes- Dominguez et al. 2008) and Ana Marcos, Agnieszka Gacek, Joseph Strauss, and David Canovas, unpublished results) and for regulation of nitrogen assimilation (Berger et al. 2008; Reyes-Dominguez et al. 2008). The strong stimulatory effect of HDAC-inhibitors on SM production in several fungi (see above) suggested a significant involvement of this histone modification in SM cluster regulation. Chromatin immunopre- cipitation at the ST locus of A. nidulans using antibodies specific for acetylated H3K9 and H3K14 provided the first direct evidence that histone acetylation is increased under secondary metabolite production conditions (Reyes-Domi- nguez et al. 2010). That indeed the catalytic and regulatory subunits GcnE and AdaB are responsible for this increase in H3 acetylation was subsequently confirmed (Nützmann et al. 2011). Our work also demonstrated that, apart from ST, other genes belonging to the PEN, TR, and ORS clusters are also targets of the Saga/Ada co-activator complex. Consistently, the ORS cluster, which is not transcribed under standard second- ary metabolite conditions, is strongly induced by addition of the HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) and induction is blocked by addition of the HAT inhibitor anacardic acid (Nützmann et al. 2011). Remarkably, severe nitrogen limitation during growth in a fully a controlled chemo- stat, induced polyketide biosynthesis genes that were otherwise silent and yielded two novel antiproliferative spiroanthrones, sanghaspirodins A and B (Scherlach et al. 2011). This would suggest that the transcriptional co-activator AreA which senses the nitrogen status of the cell and conveys this information to nitrogen metabolite repressed genes (Berger et al. 2008; Caddick et al. 1986; Scazzocchio 2000; Schinko et al. 2010), is involved in ORS cluster activation. AreA dependence has not been directly tested for any A. nidulans SM cluster, however, a direct link between nitrogen regulation and Sa- ga/Ada function has been demonstrated for primary metabo- lism in the nitrate assimilation pathway. In this model system, the GATA-factor AreA responds to intracellular glutamine (Gln) concentrations and is active under nitrogen limiting, low Gln conditions. Consequently, N-starvation or any other condition resulting in low Gln leads to histone H3 acetylation and chromatin remodeling in a strictly AreA-dependent man- ner (Berger et al. 2006; Berger et al. 2008; Muro-Pastor et al. 1999; Schinko et al. 2010). Interestingly, the production of sterigmatocystin is stimulated by NO3− and repressed by NH4+ in A. nidulans cells, and microarray data of the nitrogen re- sponse confirmed the up-regulation of genes involved in the biosynthesis of SM precursors and ST genes under NO3− conditions (Schinko et al. 2010). Unfortunately, a systematic study of AreA function in A. nidulans secondary metabolism is difficult because growth on nitrate, which is necessary for ST production, is not possible in areA loss-of-function strains.