1398 the best-studied clusters). This view is supported by the finding that bacteria-induced transcription of the ORS clus- ter is LaeA-independent (Nützmann et al. 2011) and that Saga/Ada-mediated H3 acetylation of the tested ST-cluster genes is largely independent of LaeA (Reyes-Dominguez et al. 2010). In conclusion, both Saga/Ada-mediated H3 and NuA4-mediated H4 acetylation appear to be essential for induction of several SM gene clusters but only H4 acetyla- tion seems to directly rely on functional LaeA. H3 acetyla- tion is influenced by LaeA function indirectly by defining the methylation status of H3K9. LaeA and the crosstalk to histone methylation In contrast to acetylation, which basically always reads out as an activating chromatin mark, histone methylation represents a much more complex language. Methylation of H3K9, H3K27, and H4K20 are most frequently associated with het- erochromatin and transcriptional silence whereas H3K4, H3K36, and H3K79 methylation are most often found—to- gether with hyperacetylation of H3K9—in actively tran- scribed euchromatin. Acetylation and methylation are exclusive on lysines and can never co-exist at the same time on the same residue. But this is only part of the truth: depend- ing on the degree of methylation (mon-, di-, or trimethylation) of a given lysine, whether the mark is posted on a histone residing in a promoter region or in the coding sequence, and which chromosomal location is affected, any given methyla- tion mark can either be read as an activating or repressing signal (Brosch et al. 2008; Cichewicz 2009; He and Lehming 2003; Kouzarides 2002; Zhang and Reinberg 2001) . A small window of this enormous complexity has been opened by the finding that A. nidulans LaeA greatly influen- ces the methylation state of H3K9 and subsequent occupancy of this locus by HepA. Binding of HepA to chromatin requires di- and tri-methylation of H3K9 (Bannister et al. 2001; Reyes- Dominguez et al. 2010). Furthermore, in S. pombe it was shown that SWI6 (the HP1 homologue) interacts with and recruits the H3K9 methyltransferase Clr4 (Ekwall et al. 1996). These tripartite interactions promote the establishment of ex- tended heterochromatic domains by repeated binding and methylation cycles. Consistent with the situation described in S. pombe, a lack of HepA led to loss of H3K9me3 in the ST cluster of A. nidulans, suggesting that HepA recruits ClrD, the orthologue of Clr4 in A. nidulans. The fact that loss of LaeA function results in strongly elevated H3K9me3 and HepA levels in the ST cluster and that HepA deletions partial- ly bypass the requirement for LaeA for transcription of several SM clusters, advocated a role of this regulator in heterochro- matin formation and transcriptional repression. The precise mechanism of LaeA function in this process remains uniden- tified but we already proposed that the protein could be Appl Microbiol Biotechnol (2012) 95:1389–1404 directly or indirectly involved in blocking heterochromatin formation. ClrD and HepA function could be affected and the function of histone H3K9 demethylases could be stimu- lated by LaeA. The fact that HepA deletion is only partially remediating a laeA mutant phenotype for ST gene transcrip- tion and metabolite production (Reyes-Dominguez et al. 2010) suggests that LaeA and HepA function to some extent in the same pathway but LaeA function must reach beyond counteracting HepA. Two putative H3K9 demethylases are present in the A. nidulans genome (our unpublished observa- tion) and one or both proteins could cooperate with LaeA to demethylate H3K9me3 within SM clusters under the specific metabolic conditions. Preliminary evidence from our labora- tory suggests that both A. nidulans KDMs are functional and play a role in SM regulation (Agnieszka Gacek and Joseph Strauss, unpublished observations). Does heterochromatin also regulate SM in other fungi? Beside from what has been described above for A. nidulans and A. parasiticus, chromatin in relation to SM gene ex- pression has been studied only in a few other organisms. It should be noted here that heterochromatin formation in filamentous fungi is without doubt best understood in N. crassa (Rountree and Selker 2010), but this organism is not a suitable model for secondary metabolism. In contrast to LaeA, which is well studied at the genetic, genomic, and physiological level and known to regulate chemical diversi- ty in a large range of fungi (Bok and Keller 2004; Kale et al. 2007; Kale et al. 2008; Kosalkova et al. 2009; Oda et al. 2011; Perrin et al. 2007; Wiemann et al. 2010; Xing et al. 2010), factors regulating heterochromatin formation in SM gene clusters have only been studied in the human opportu- nistic pathogen Aspergillus fumigatus and the plant patho- gen Fusarium graminearum. Deletion of the A. fumigatus H3K9 methyltransferase ClrD reduced viability and conidia production but did not change host interactions in a macro- phage assay (Palmer et al. 2008). The effect of clrD deletion on chromatin structure, histone marks, and secondary me- tabolism has not been reported yet for A. fumigatus, In contrast, a clear effect on secondary metabolism is seen in the heterochromatin protein 1 (hep1) deletion strains of the plant pathogen F. graminaerum (Reyes-Dominguez et al. 2012). Similar to A. nidulans hepA deletion strains, H3K9me3 is strongly reduced in the F. graminaerum hep1 mutants in two examined gene clusters (deoxynivale- nol-DON) and aurofusarin AUR pigment). As expected, the absence of HEP1 and H3K9me3 leads to the activation of the aurofusarin gene cluster and consequently strongly ele- vated metabolite production. Surprisingly, the same muta- tion leads to gene repression of two genes belonging to the DON cluster and concomitant absence of the corresponding