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Development 129, 2483-2493 (2002)
© 2002 The Company of Biologists Limited

The Drosophila Trithorax protein is a coactivator required to prevent re-establishment of Polycomb silencing

Sylvain Poux, Béatrice Horard*, Christian J. A. Sigrist{dagger} and Vincenzo Pirrotta{ddagger}

Department of Zoology, University of Geneva, 30 quai Ernest Ansermet, CH-1211 Geneva, Switzerland
* Present address: LBMC-ENS Lyon, 46, allée d’ Italie, F-69364 Lyon Cedex 07, France
{dagger} Present address: Swiss Institute of Bioinformatics (SIB-ISB), 1, rue Michel Servet, CH-1211 Geneva, Switzerland

{ddagger}Author for correspondence (e-mail: vincenzo.pirrotta{at}zoo.unige.ch)

Accepted 1 March 2002

Polycomb group (PcG) and Trithorax (TRX) complexes assemble at Polycomb response elements (PREs) and maintain respectively the repressed and active state of homeotic genes. Although PcG and TRX complexes are distinct, their binding to some PRE fragments in vitro depends on GAGA motifs. GAGA factor immunoprecipitates with both complexes. In presence of a PRE, TRX stimulates expression and prevents the return of repression at later stages. When TRX levels are reduced, repression is re-established in inappropriate regions of imaginal discs, suggesting that TRX insufficiency impairs the epigenetic memory of the active state. Targeting a GAL-TRX fusion shows that TRX is a coactivator that stimulates expression of an active gene but cannot initiate expression by itself. Targeting a histone acetylase to a PRE does not affect embryonic silencing but causes a loss of memory in imaginal discs, suggesting that deacetylation is required to establish the memory of the repressed state.

Key words: Chromatin silencing, Polycomb, Coactivator, Homeotic genes, Epigenetic memory, Drosophila melanogaster




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© The Company of Biologists Ltd 2002