Histone Genetics in Drosophila.

Histone Genetics in Drosophila. Jürg Müller^1*, Ana Raquel Penelly¹, Omer Copur¹, Katja Finkl¹, Herbert Jäckle², Alf Herzig². 1) Max-Planck Institute of Biochemistry, Am Klopferspitz 18, 82152 Martinsried, Bavaria, Germany; 2) Max-Planck Institute for Biophysical Chemistry, Molecular Developmental Biology, Am Fassberg 11 37077 Göttingen, Germany.

We are interested in understanding the molecular mechanisms that permit cell-specific transcription and repression of developmental regulator genes to be established and maintained. To this end, we have been studying the Polycomb/trithorax system. Polycomb group (PcG) proteins control a variety of cell fate decisions in animals and plants by repressing developmental regulator genes in cells where they should not be expressed. In Drosophila, biochemical studies established that PcG proteins exist in four principal protein complexes: PRC1, PRC2, PhoRC and PR-DUB. All four complexes are bound at PcG target genes in vivo and are thought to repress their transcription by modifying chromatin. The tri-methylation of histone H3 at lysine 27 (H3-K27me3) by PRC2 and the monoubiquitylation of histone H2A (H2A-ub1) at K118 in Drosophila and K119 in mammals by PRC1-type complexes are thought to be central to the repression mechanism of the Polycomb system. We have been investigating the function of post-translational modifications on histones by performing histone genetics in Drosophila. To this end, we generated Drosophila strains in which the endogenous histone genes can be conditionally removed and replaced with transgenes encoding histone proteins with point mutations. We found that cells containing H3-K27R instead of H3-K27 fail to repress PRC2 target genes and reproduce the PRC2 mutant phenotype. This demonstrates that H3-K27 is the crucial physiological substrate that PRC2 needs to modify for Polycomb repression. However, the analysis of H2A mutants that can no longer be ubiquitylated and of other histone point mutants resulted in several unexpected findings. Progress on these studies will be presented.