ths is directly activated by low levels of the Dorsal gradient in the neuroectoderm (Stathopoulos, 2002). ths is probably kept off in the ventral mesoderm by the localized Snail repressor because the neuroectoderm enhancer contains an optimal Snail-binding site. It is conceivable that the Dorsal gradient controls mesoderm spreading by differentially regulating the ths/pyr FGF ligands in the neuroectoderm and the FGF receptor and intracellular signaling components in the mesoderm (Stathopoulos, 2004).
To investigate this possibility, computational methods were used to identify putative Dorsal target enhancers for the htl and dof/hbr/sms genes. Both genes are activated in the presumptive mesoderm prior to the formation of the ventral furrow, and both are required for the spreading of the mesoderm after invagination. A survey of the htl locus identified a cluster of two putative Dorsal-binding sites and two copies of a distinct sequence motif, CACATGT, which probably binds the Twist activator and is found in several Dorsal target enhancers (Stathopoulos, 2002). The Dorsal-Twist binding cluster is located within the first intron of the htl gene. When expressed in transgenic embryos, this 800-bp fragment directs lacZ expression in the ventral furrow and invaginated mesoderm. A putative dof/hbr/sms enhancer was identified within the first intron of this gene as a cluster of two Dorsal-binding sites and a copy of a conserved sequence motif, RGGNCAG, which is seen in a variety of Dorsal target enhancers (Stathopoulos, 2002). When attached to the lacZ reporter gene, this cluster directs weak expression in the mesoderm of early embryos and tracheal pits of older embryos (Stathopoulos, 2004).
These results provide evidence that htl and dof/hbr/sms are direct target genes of the Dorsal gradient that are induced in response to peak levels of nuclear Dorsal present in ventral regions of early embryos. The previously identified ths enhancer (previously called the Neu4 enhancer) contains three high-affinity Dorsal-binding sites and a Snail repressor site (Stathopoulos, 2002). The ths enhancer directs expression throughout the neurogenic ectoderm during early stages of gastrulation in response to lower levels of nuclear Dorsal. It is, therefore, possible to describe gastrulation as a series of discrete threshold readouts of the Dorsal gradient (Stathopoulos, 2004).
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