Heterochromatin is a repressive chromatin structure involved in gene silencing phenomena. One of the principal components of heterochromatin formation and maintenance is the heterochromatin protein 1, HP1 (or SWI6 in S .pombe) which requires lysine 9 methylation marks on histone H3 (H3K9met) to bind to chromatin and induce a repressive heterochromatin structure. The deletion of the unique HP1 homologous gene, hepA in A. nidulans, causes the deregulation of several genes related to secondary metabolism, including genes belonging to the sterigmatocystin (ST) biosynthetic cluster. The deletion of hepA leads to an earlier production and higher accumulation of ST. We show by chromatin immunoprecipitation (ChIP) analysis on the promoter region of aflR, the gene encoding the specific ST cluster transcriptional activator that HepA occupies the aflR locus during transcriptional silencing of the cluster but dissociates from the promoter during production of ST. The amount of HepA present at the aflR promoter directly correlates with the level of lysine-9 trimethylation of histone H3. In A. nidulans, LaeA was shown to be required for transcription of ST and penicillin clusters. We show that deletion of hepA or the H3K9 methylase clrD suppresses the laeA916; phenotype and that production of secondary metabolites is restored in a double laeA916;/hepA916; or laeA916;/clrD916; strain. Our ChIP results using the single and double mutant strains suggest that LaeA counteracts binding of HepA to H3K9me in the aflR promoter region under conditions of ST production. We show by Micrococcal nuclease (MNase) chromatin analysis that, in a wild type strain, MNase accessibility of the ST locus is restricted during conditions of primary metabolism suggesting the formation of heterochromatic structures. In contrast, at the onset of ST cluster activation MNase access increases indicating a reversal of silencing by euchromatization of the ST locus. Additionally, the effect of the deletion of hepA and laeA on heterochromatin/euchromatin switching of ST locus is presented. Our results indicate that facultative heterochromatin structures regulate the timing of expression and overall activity of the ST-gene locus in A. nidulans and that H3K9me and subsequent HepA binding determines locus silencing during primary metabolism. LaeA, the general positive regulator of secondary metabolism is directly involved in reversing this repressive chromatin structure thus contributing to cluster activation at the onset of secondary metabolism.
Full conference title:
9th EUROPEAN CONFERENCE ON FUNGAL GENETICS
- ECFG 9th (2008)