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Fig. 3. Transcriptional regulation by Yki and Sd. In Drosophila,
Yorkie (Yki) and Scalloped (Sd) form a heterodimeric transcription factor that
regulates downstream targets of Warts signaling. Their mammalian homologues
Yap and Tead/Tef1-Tef4 (not shown) perform a similar function in mammalian
cells. Genetic studies in Drosophila indicate that yki
mutation reduces organ growth, whereas sd mutation has little effect
outside of the wing. Two possible explanations (which are not mutually
exclusive) for this are proposed. (A) In the absence of Yki, target
genes might be actively repressed by Sd (right image), presumably in concert
with, as yet, unidentified repressors (Rep). Target genes would be expressed
at modest levels (thin red line) in the absence of Sd (owing to derepression),
but would not to be expressed at all in the absence of Yki. (B)
Alternatively, Yki might complex with other DNA-binding proteins (X). These
other complexes could then act independently of Sd to promote the expression
of the same downstream target genes. In this case, partial expression of
targets would occur in the absence of Sd, but not in the absence of Yki.
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