XB-ART-10981Development June 1, 2000; 127 (12): 2729-39.
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Region-specific activation of the Xenopus brachyury promoter involves active repression in ectoderm and endoderm: a study using transgenic frog embryos.
Tissue specification in the early embryo requires the integration of spatial information at the promoters of developmentally important genes. Although several response elements for signalling pathways have been identified in Xenopus promoters, it is not yet understood what defines the sharp borders that restrict expression to a specific tissue. Here we use transgenic frog embryos to study the spatial and temporal regulation of the Xbra promoter. Deletion analysis and point mutations in putative transcription factor-binding sites identified two repressor modules, which exert their main effects at different stages during gastrulation. One module is defined by a bipartite binding site for a Smad-interacting protein (SIP1) of the deltaEF1 repressor family and acts to confine expression to the marginal zone early in gastrulation. The other module is defined by two homeodomain-binding sites and is responsible for repression in dorsal mesoderm and ectoderm at mid-gastrula stages. In addition, an upstream region of the promoter is necessary to repress expression in neural tissues later in development. Together, our results show that repression plays an important role in the restriction of Xbra expression to the mesoderm, and we suggest that similar mechanisms may be involved in the spatial regulation of other genes in early embryonic development.
PubMed ID: 10821770
Article link: Development
Species referenced: Xenopus
Genes referenced: tbx2 tbxt zeb1
Article Images: [+] show captions
|Fig. 1. Comparison of Xbra reporter gene expression with that of the endogenous gene. (A) Time course of Xbra-2.1 expression compared with that of endogenous Xbra. Dorsal is upwards in all panels, except wild-type stage 9.5 (dorsal to the right). The colour reaction to detect reporter gene expression took 24 to 48 hours compared with 5 hours to detect endogenous Xbra. Reporter gene expression was restricted to the marginal zone in approximately 80% (n>150) of transgenic embryos. (B) Expression of endogenous Xbra RNA at stage 9.0. Left panel: animal pole view; right panel: side view. Embryos were cleared to visualise internal staining. Nuclear staining indicates newly transcribed zygotic RNA. Non-transgenic embryos stained for GFP RNA for the same time showed no expression (not shown). Weaker staining in the vegetal pole maybe due to poor probe diffusion, but see the sectioned in situ hybridisations of Panitz et al. (1998). (C) GFP fluorescence of an embryo transgenic for Xbra-4.1. Note slightly weaker expression in the dorsal marginal zone (top right).|