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activity is required for the positioning of the mesoderm/neuroectoderm boundary in Xenopus1 Adolf-Butenandt-Institut, Institut für Molekularbiologie, Ludwig-Maximilians-Universität, 80336 München, Germany; 2 GSF-Forschungszentrum, Institut für Molekulare Immunologie, 81377 München, Germany
Experiments in Xenopus have illustrated the importance of extracellular morphogens for embryonic gene regulation in vertebrates. Much less is known about how induction leads to the correct positioning of boundaries; for example, between germ layers. Here we report that the neuroectoderm/mesoderm boundary is controlled by the chromatin remodeling ATPase CHD4/Mi-2
. Gain and loss of CHD4 function experiments shifted this boundary along the animalvegetal axis at gastrulation, leading to excess mesoderm formation at the expense of neuroectoderm, or vice versa. This phenotype results from specific alterations in gene transcription, notably of the neural-promoting gene Sip1 and the mesodermal regulatory gene Xbra. We show that CHD4 suppresses Sip1 transcription by direct binding to the 5' end of the Sip1 gene body. Furthermore, we demonstrate that CHD4 and Sip1 expression levels determine the "ON" threshold for Nodal-dependent but not for eFGF-dependent induction of Xbra transcription. The CHD4/Sip1 epistasis thus constitutes a regulatory module, which balances mesoderm and neuroectoderm formation.
[Keywords: CHD4/Mi-2
; Sip1; Xbra; chromatin remodeling; Activin induction threshold; germ layer]
Received September 12, 2006; revised version accepted February 23, 2007.
E-MAIL ralph.rupp{at}med.uni-muenchen.de; FAX 49-89-2180-75-440.
Supplemental material is available at http://www.genesdev.org.
Article is online at http://www.genesdev.org/cgi/doi/10.1101/gad.409507
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