In multicellular organisms, emergence of different cell types, despite sharing the same set of genetic information, is achieved very early during development and is orchestrated by spatio-temporal gene expression programs. Precise specification of cell fate, resulting from establishment of differential gene expression, within a cell lineage relies on faithful inheritance of cell type specific gene expression patterns through successive cell divisions, also referred to as epigenetic cell memory. Genetic and molecular analysis in Drosophila has established that evolutionary conserved Polycomb group (PcG) and trithorax group (trxG) genes maintain cellular identities by maintaining differential gene expression patterns. Initially discovered as positive regulators of HOX genes in forward genetic screens, trxG counteracts PcG mediated repression of cell type specific genes. Despite decades of extensive analysis, molecular understanding of trxG action and regulation is still punctuated by many unknowns. This study is aimed at characterizing Drosophila Enok which was identified as a trxG gene in a genome-wide RNAi screen in our laboratory. Importantly, Enok was also previously purified with PRC1 (Polycomb repressive complex 1). The presence of Enok among trxG candidates in genome-wide RNAi screen as well as in PRC1 complex raises the question how it may contribute to either gene activation or repression, respectively. This dissertation describes my quest to unravel the mysterious role of Enok during the process of epigenetic cell memory. Enok is a known acetyltransferase that forms quartet complex (Enok, Br140, ING5 and Eaf6) and is involved in acetylation of histone H3 at lysine 23 (H3K23ac).
Genetic analysis described here reveals that mutants of both enok and Br140 strongly suppressed extra sex comb phenotype of Pc mutants and enhanced homeotic transformations associated with trx mutations. This trxG like behavior of Enok was further substantiated by the fact that depletion of either enok or Br140 with concomitant loss of H3K23ac resulted in reduced expression of trxG target genes. This downregulation of trxG targets was accompanied by an increased enrichment of PC and mono-ubiquitination of histone H2A at lysine 118 which is a known hallmark of PcG. Moreover, depletion of Enok also resulted in an increased stalling of RNA Pol-II in the promoter region of trxG target genes. Although Enok was found to colocalize with both TRX and PC at the chromatin, it was discovered that Enok specifically counteracts PRC1 mediated repression. Together, genetic and molecular analysis described in this dissertation demonstrates that Enok complex specifically contributes to the maintenance of gene activation by counteracting PcG. Further, molecular and biochemical characterization of H3K23ac by Enok and its cross-talk with covalent modifications of histones catalyzed by trxG and PcG will help understand how it is required for maintenance of gene activation by trxG.