Abstract
In Drosophila, PcG complexes provide heritable transcriptional silencing of target genes. Among them, the ESC/E(Z) complex is thought to play a role in the initiation of silencing whereas other complexes such as the PRC1 complex are thought to maintain it. PcG complexes are thought to be recruited to DNA through interaction with DNA binding proteins such as the GAGA factor, but no direct interactions between the constituents of PcG complexes and the GAGA factor have been reported so far. The Drosophila corto gene interacts with E(z) as well as with genes encoding members of maintenance complexes, suggesting that it could play a role in the transition between the initiation and maintenance of PcG silencing. Moreover, corto also interacts genetically with Trl, which encodes the GAGA factor, suggesting that it may serve as a mediator in recruiting PcG complexes. Here, we show that Corto bears a chromo domain and we provide evidence for in vivo association of Corto with ESC and with PC in embryos. Moreover, we show by GST pull-down and two-hybrid experiments that Corto binds to E(Z), ESC, PH, SCM and GAGA and co-localizes with these proteins on a few sites on polytene chromosomes. These results reinforce the idea that Corto plays a role in PcG silencing, perhaps by confering target specificity.
Highlights
The power of Drosophila genetics has permitted the discovery of genes encoding proteins involved in epigenetic mechanisms before knowing the molecular structure of chromatin
The GAGA protein contains a BTB/POZ domain (BR-C, Tramtrack, Bab, Pox virus, Zinc ®nger), a Previous studies have suggested that Corto acts as a chromatin structure modulator involved in Polycomb group (PcG) silencing [22,23]
The present results provide molecular evidence supporting the fact that this protein exhibits a chromo domain shared with some chromatin binding proteins, participates in PcG complexes and directly contacts some PcG proteins as well as the GAGA factor
Summary
The power of Drosophila genetics has permitted the discovery of genes encoding proteins involved in epigenetic mechanisms before knowing the molecular structure of chromatin. Genetic screens identi®ed two classes of regulatory factors responsible for maintaining homeotic (Hox) gene expression throughout development. These factors are encoded by two groups of genes: the Polycomb group (PcG) and the trithorax group (trxG). The PcG and TrxG proteins were later on shown to function as multimeric complexes that regulate Hox gene expression through modulation of the chromatin structure. In Drosophila, the domains of Hox gene expression are established early during development by the gap and pair rule gene products. The pleiotropic phenotypes of PcG and trxG Drosophila mutants and the localization of the proteins they encode at multiple sites on polytene chromosomes were the ®rst indications that the function of these regulators was not restricted to the Hox genes. Several non-Hox targets were further discovered and key genes, such as segmentation genes or dorso±ventral polarity genes, were shown to be targets for PcG [2,3] or TrxG [4,5] factors
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