VegT activation of the early zygotic gene Xnr5 requires lifting of Tcf-mediated repression in the Xenopus blastula.
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.
Hilton
Emma
E
Biomolecular Medicine Group, Department of Biological Sciences, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK.
Rex
Maria
M
Old
Robert
R
D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D036703:Blastula / Q000378:metabolism*; D003598:Cytoskeletal Proteins / Q000378:metabolism; D017930:Genes, Reporter; D024002:HMGB Proteins / Q000378:metabolism; D008969:Molecular Sequence Data; D009154:Mutation; D055456:Nodal Signaling Ligands; D011401:Promoter Regions, Genetic; D020825:T-Box Domain Proteins / Q000378:metabolism*; D051820:TCF Transcription Factors; D015534:Trans-Activators / Q000378:metabolism; D058127:Transcription Factor 3; D058125:Transcription Factor 7-Like 1 Protein; D014157:Transcription Factors / Q000378:metabolism; D014981:Xenopus; D029867:Xenopus Proteins / Q000235:genetics* / Q000378:metabolism; D051176:beta Catenin
C495266:CTNNB1 protein, Xenopus; D003598:Cytoskeletal Proteins; D024002:HMGB Proteins; D055456:Nodal Signaling Ligands; D020825:T-Box Domain Proteins; D051820:TCF Transcription Factors; C544917:Tcf3 protein, Xenopus; D015534:Trans-Activators; D058127:Transcription Factor 3; D058125:Transcription Factor 7-Like 1 Protein; D014157:Transcription Factors; C103966:VegT protein, Xenopus; D029867:Xenopus Proteins; D051176:beta Catenin; C418882:nodal5 protein, Xenopus