UNSPECIFIED. (2003) VegT activation of the early zygotic gene Xnr5 requires lifting of Tcf-mediated repression in the Xenopus blastula. MECHANISMS OF DEVELOPMENT, 120 (10). pp. 1127-1138. ISSN 0925-4773

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Abstract

Xenopus Nodal-related (Xnr) 5 is one of the earliest expressed components of a network of TGF-beta factors participating in endoderm and mesoderm formation. Zygotic gene expression is not required for induction of Xnr5; rather, expression is dependent on the maternal factors VegT, localised throughout the vegetal pole, and beta-catenin, functional in the future dorsal region of the embryo. Using transient assays with a luciferase reporter in Xenopus embryos, we have defined a minimal promoter, which mimics the response of the endogenous gene to applied factors. Expression of luciferase from the minimal promoter is dorsal-specific and requires two T-box half sites and a functional beta-catenin/XTcf-3 pathway. Mutation of two Tcf/Lef sites in the minimal promoter permits induction by VegT to wild-type promoter levels in the presence of a dominant-negative XTcf-3, indicating that beta-catenin/XTcf-3 are repressive and are not required as transactivators of Xnr5 transcription. The activity of the Tcf/Lef mutant promoter is similar in both ventral and dorsal sides of the embryo. In transgenic experiments, the dorsal specificity of expression of a beta-gal reporter driven by the wild-type minimal promoter is abolished upon mutation of these Tcf/Lef sites. We propose a model in which XTcf-3 functions as a repressor of Xnr5 throughout the blastula, embryo, except where repression is lifted by the binding of beta-catenin in the dorsal region. This removal of repression allows activation of the promoter by VegT in the dorsal vegetal region. Subsequently, zygotically expressed LEF1 supersedes the role of beta-catenin/XTcf-3. (C) 2003 Elsevier Ireland Ltd. All rights reserved.

Item Type:	Journal Article
Subjects:	Q Science > QH Natural history > QH301 Biology
Journal or Publication Title:	MECHANISMS OF DEVELOPMENT
Publisher:	ELSEVIER SCIENCE BV
ISSN:	0925-4773
Date:	October 2003
Volume:	120
Number:	10
Number of Pages:	12
Page Range:	pp. 1127-1138
Identification Number:	10.1016/j.mod.2003.08.004
Publication Status:	Published
URI:	http://wrap.warwick.ac.uk/id/eprint/9148

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