Figure 6. Model for Hedgehog/Tbx5 interaction.

(A) Intersection of Tbx5 expression, restricted to the posterior SHF and heart, and Gli1 expression, broadly expressed in axial mesenchyme and brain but excluded from the heart, is the posterior SHF. Activation of TBX5/GLI1 responsive enhancer is observed principally in the overlap between the Tbx5 and Gli1 expression domains. (B) In the presence of GLI activator (GLIA) alone, the enhancer is weakly active. In the presence of both GLIA and TBX5 is transcription from the enhancer strongly activated. When the GLI binding site is mutated, GLIA alone is insufficient to activate strong expression, but GlLIA may interact with TBX5 to activate expression more strongly than TBX5 alone.