Analysis of the mechanism of maintenance of the H3K27 trimethylation mark using a novel chromatin targeting system

Chromatin replication during cell division must be accurately orchestrated to ensure genetic and epigenetic information is transmitted to cell progeny. Upon cell division, newly synthesized histones assemble onto the newly formed chromatin to replace the disassociated parental histones. As these newly synthesized histones do not contain the same post-translational modifications as their adjacent parental histones, these modifications must be recapitulated after each cell division. The trimethylation of lysine 27 (K27me3) on histone H3 is associated with transcriptional repression, and is deposited by EZH2, a member of the PRC2 complex. Using a Gal4 DNA binding domain (Gal4DBD) fused to EZH2 coupled with FLP/FRT-based deletion of a gal4 binding site cassette, I provide evidence that, once established, the maintenance of H3K27me3 does not require the presence of the DNA binding sites necessary for the initial deposition of this mark. These results suggest that the presence of specific histone marks may be sufficient to promote reiterative deposition of the same mark on nascent histones in association with DNA replication.Medicine, Faculty ofMedical Genetics, Department ofGraduat