Fig. 5. SSF is dominant negative for high signaling. All tissue (except L) is from MS1096 hemizygotes. (A-E,M,N) Heterozygous smo3 UasSSF^high, (F-I,O,P) heterozygous smo3 UasSSF^high; Su(fu^)LP, (J) smo³ UasSSF^high/UasFu, (K) smo³ UasSSF^high/UasSmo. (C,G) Expression of dpp, (D,H) ptc, (E,I) col and (L-P) Ci C-terminal epitope. SSF gave strong narrowing of L3/4 (bracket in A). This reflected loss of col (E) and ptc (D), and expansion of dpp (C) and Iro (bracket in B). In the notum primordium, where MS1096 does not drive transgene expression (bracket in M,N), and in wild-type imaginal discs (L), Ci155 accumulated in a zone some four or five cells wide. The apparent fading of Ci155 in the posterior-most two or three cells of its domain (immediately adjacent to the compartment border) is due to Hh-dependent depletion (e.g. Ohlmeyer and Kalderon, 1998). With SSF, Ci155 accumulated in a broad domain with no depletion at the compartment border (M,N). If Ci155 is free to enter nuclei, it is trapped there by the nuclear export blocker, LMB (N-P). In wild type, Ci155 was trapped in nuclei of six or seven cells adjacent to the border, but not deeper in the anterior compartment (bracket in N). SSF blocked nuclear accumulation of Ci155 at the border (remainder of N). SSF did not affect the L3/4 intervein in a Su(fu) mutant background (bracket in F), and instead gave anterior expansion of L3 and overgrowth along the AP axis. ptc (H) and col (I) were partly rescued at the compartment border, while dpp (G) and ptc expanded across the overgrown dorsal anterior compartment. Ci155 accumulated in nuclei of LMB-treated Sufu mutant discs, both at the border (P) and deep in the anterior compartment (O). The L3/4 narrowing of SSF was mildly suppressed by simultaneous expression of Fu (J) and was more strongly suppressed by simultaneous overexpression of Smo (K).