Regulation of Homeotic Genes in Drosophila
HOX genes are involved in diversification of body segments along anteroposterior axis in animal kingdom. They were initially identified and are most extensively characterized in Drosophila. The temporal and spatial expression of HOX genes must be precisely controlled to maintain proper development of the body. The underlying mechanisms remain elusive.
Two groups of trans-acting genes are involved in maintenance of the "on-off" state of HOX genes: trithorax group (trx-G) activators and Polycomb group (Pc-G) repressors. Many of these factors appear to form large protein complexes to affect promoter activity through novel mechanisms involving modifications or remodeling of chromatin organization. Our goal is to define these molecular events, using a HOX gene Ultrabithorax (Ubx) as a model system.
We have recently adopted a step-wise approach to purify protein complexes, characterize their biochemical activities, identify their molecular entities, followed by in vivo analyses of their roles during animal development. So far, we have purified several complexes from Pc-G and extensively characterized one such complex we named CHRASCH (CHRomatin-Associated Silencing Complex for Homeotics). We found that CHRASCH contains several novel activities relevant for homeotic gene regulation. For examples, a DNA binding factor encoded by pipsqueak was found in CHRASCH and is apparently essential for sequence-specific targeting of CHRASCH. This finding may partly resolve a fundamental question about the targeting of Pc-G. In addition, we identified histone deacetylase 1 as a functional constituent of CHRASCH. We are currently investigating several other novel activities of CHRASCH. As these factors are involved in modification of histones, we anticipate that detailed knowledge of the function of these factors will allow us to decipher the "histone code" created by CHRASCH.
We have also carried out extensive analysis of female sterile homeotic (fsh), a trx-G activator. fsh encodes two related proteins containing double bromodomain. The large isoform is a novel centrosomal protein essential for integrity of mitotic apparatus during early embryonic development. The small isoform appears to be a transcription regulator acting on the proximal promoter of Ubx through a critical proximal region. Our recent results suggest that this small isoform may mediate long range interaction between distal enhancers and the basal promoter.