Identification of a conserved cis-acting region driving expression of mouse Eomesodermin to the primitive streak, node, and definitive endoderm.

The cis-acting elements that regulate Eomes transcription during embryonic development are largely unknown. Here we identify a conserved cis-acting region (EoIV) located ~20kb upstream of the Eomes coding region that faithfully drives reporter expression to sites of Eomes expression during gastrulation. Transgenic EoIV-hsp68-GFP expression was evident in the epiblast of early-streak stage mouse embryos at the site where the primitive streak is initiated. At the mid- and late-streak stages, EoIV-hsp68-GFP expression was found in the streak, node region and definitive endoderm with a particular intensive GFP expression in the node region. At the early head fold stage, GFP was expressed in the node region and the surrounding endoderm. In contrast to earlier reports of Eomes mRNA expression, we confirmed Eomes protein expression in the node of early head fold embryos by immunohistochemistry. In vitro, EoIV-hsp68-GFP expression was activated ES cells differentiating into primitive streak-like progeny in response to Bmp and activin treatment.