Dynamic regulation of Brachyury expression in the amphibian embryo by XSIP1

Xenopus Brachyury (Xbra) plays a key role in mesoderm formation during early development. One factor thought to be involved in the regulation of Xbra is XSIP1, a zinc finger/homeodomain-like DNA-binding protein that belongs to the deltaEF1 family of transcriptional repressors. We show here that Xbra and XSIP1 are co-expressed at the onset of gastrulation, but that expression subsequently refines such that Xbra is expressed in prospective mesoderm and XSIP1 in anterior neurectoderm. This refinement of the expression patterns of the two genes is due in part to the ability of XSIP1 to repress expression of Xbra. This repression is highly specific, in the sense that XSIP1 does not repress the expression of other regionally expressed genes in the early embryo, and that other members of the family to which XSIP1 belongs, such as deltaEF1 and its Xenopus homologue ZEB, cannot regulate Xbra expression. The function of XSIP1 was studied further by making an interfering construct comprising the open reading frame of XSIP1 fused to the VP16 transactivation domain. Experiments using this chimeric protein suggest that XSIP1 is required for normal gastrulation movements to occur and for the development of the anterior neural plate.