Fig. 5. Differential requirement of STAT for RTK signaling. The RTK Tor induces the expression of its target gene tll by derepression via activating the Ras-MAPK signaling pathway. Additional yet unidentified activators (A) and repressors (R₂), which may or may not be controlled by Tor, determine the transcription levels of tll in a combinatorial manner. The activators (A) and repressors (R₂) are unevenly distributed in cells along the anteroposterior axis of the embryo. In the central region of the embryo, there are higher levels of the repressors or lower levels of activators than the posterior region (only one possibility is shown). Mrl (STAT) is not essential for tll expression under wild-type conditions. However, Tor^GOF activates Mrl. Activated Mrl is required to overcome the higher levels of repressors (R₂) in tissues where tll is not normally expressed, resulting in developmental abnormalities. Arrow and bar indicate activation and repression, respectively. Dotted lines represent undermined events.

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