Fig. 7

Model for the differential constraint on FoxF vs. other early TVC enhancers. a Before FGF induction, the chromatin around early TVC gene enhancers is condensed preventing aberrant expression. In contrast, chromatin is decondensed at the FoxF TVC enhancer locus, suggesting that a repressor (purple) is required to prevent precocious expression. b FGF/MapK-signaling phosphorylates Ets1/2 in the TVCs, permitting recruitment of a co-factor (green) that serves to lift repression. The cooperative recruitment of this co-factor constrains binding site position and orientation. FoxF (orange) then accumulates in the TVC nuclei, where it acts as a pioneer factor opening the chromatin around other TVC enhancers. c Once early TVC gene enhancers are open, the binding of Ets1/2, ATTA, and FoxF activates transcription in a non-cooperative fashion, as reflected by a lack of constraint on binding site position. FoxF also binds the FoxF TVC enhancer helping to maintain its own expression