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Fig. 4. Tcf3 is predominantly required as a transcription repressor in mesoderm induction, while Tcf1 and Lef1 are predominantly required as transcription activators in mesoderm development. (A) Schematic representation of mutated XlTcf3 constructs used as molecular tools in this study. The ß-catenin binding domain, the DNA-binding HMG box, and the Grg- and CtBP-binding domains are as indicated. Tcf3{Delta}N represents a constitutive repressor form, Tcf3{Delta}grg{Delta}C a ß-catenin-dependent active form and TVGR a constitutive activator form of XlTcf3. (B-I) Vegetal view of brachyury (Xbra) expression at stage 10.5 embryos, dorsal towards the top, injections into the right side. Blocking of Xbra expression by Tcf1 MO (B) is hardly rescued by co-injection of 0.3 ng Tcf3{Delta}N mRNA (C), but is significantly rescued by co-injection of 0.3 ng Tcf3{Delta}grg{Delta}C mRNA (D) or 1 pg TVGR mRNA (induced at stage 9) (E). Downregulation of Xbra expression by XlTcf3 MO (F) is rescued by co-injection 0.3 ng HA-XTcf3{Delta}N mRNA (G) but is hardly rescued by co-injection of 0.3 ng HA-XTcf3{Delta}grg{Delta}C mRNA (H) or 1 pg TVGR mRNA (induced at stage 9) (I). (J-Q) Vegetal view of Xenopus posterior (Xpo) expression at stage 10.5 embryos, dorsal towards the top, injections into the right side. Downregulation of Xpo expression by Lef1 MO (J) is not rescued by co-injection of 0.3 ng XTcf3{Delta}N mRNA (K), but is rescued by co-injection of 0.3 ng XTcf3{Delta}grg{Delta}C mRNA (L) or 1 pg TVGR mRNA (M, induced at stage 9). Similarly, downregulation of Xpo expression by Tcf1 MO (N) is not rescued by co-injection of 0.3 ng XTcf3{Delta}N mRNA (O), but is rescued by co-injection of 0.3 ng XTcf3{Delta}grg{Delta}C mRNA (P) or 1 pg TVGR mRNA (Q, induced at stage 9). (R) Numerical summary illustrating the penetrance of rescue effects of Tcf3 mutants mRNAs with distinct activity for downregulation of Xbra expression caused by Tcf1/Tcf3 MOs, indicating absent and reduced Xbra expression at the site of injection. (S) Numerical summary illustrating the penetrance of rescue effects of mRNAs encoding different Tcf3 mutated constructs in Tcf1 MO- or Lef1 MO-injected embryos, indicating absent or reduced Xpo expression at the site of injection.