Epigenetic regulation of active chromatin’

• Applicant: Professor Dr. Peter Burkhard Becker
• Subject Area: General Genetics and Functional Genome Biology ; Biochemistry
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 452337045

The compensation for X chromosome monosomy in Drosophila serves as an instructive example for epigenetic regulation of active chromatin. Dosage compensation in flies adjusts the transcription of the single X chromosome in males by roughly 2-fold activation to match the combined output of the two female X’s. Central to this regulation is the Dosage Compensation Complex (DCC), which consists of 5 male-specific-lethal protein subunits and non-coding roX RNA.According to the prevalent model, the exclusive activation of genes on the X chromosome involves genetic and epigenetic principles. First, the DCC selectively binds to some 300 DNA sequence-defined ‘High Affinity Sites’ (HAS) on the X. From there it ‘reaches out’ to epigenetically marked target genes to boost their transcription through histone acetylation. This proposal addresses unsolved questions about the nature of the epigenetic chromatin modification and the ‘reader’ and ‘writer’ functions of the DCC that allows it to selectively acetylate active chromatin. The current model states that DCC bound to a HAS searches the chromosomal neighborhood for transcribed chromatin, using the chromodomain of the MSL3 subunit as a ‘reader head’. This domain can bind nucleosomes marked by methylation of histone H3 at lysine 36 (H3K36me3), a modification that is placed co-transcriptionally. MSL3 is connected to the acetyltransferase MOF through the scaffold protein MSL1. Accordingly, the MSL3-nucleosome interaction recruits MOF to active chromatin, where it acetylates H4K16. This acetylation is thought to facilitate transcription through chromatin unfolding. We wish to critically evaluate this model and explore the principles that allow regional H4K16 acetylation in the chromosome. Our biochemical approach is aimed at deciphering molecular mechanism.Reinforcing feedback loops are better known for the maintenance of repressive chromatin structures, such as constitutive and facultative heterochromatin. Our study addresses an unusual case, where features of active chromatin trigger additional activation. The epigenetic reader in the DCC (a methyllysine binder) and the writer enzyme (a histone acetyltransferase) exemplify widely used, evolutionary conserved regulatory molecules. Knowledge gained from our study will have wide implications beyond the specific case of sex chromosome regulation in flies.

• DFG Programme: Research Grants