Fig. 1. An Xvent2-BRE reporter mimics brk expression in Drosophila and assembles a Mad/Med/Shn protein-DNA complex. (A) Reporter constructs. (B-Q) Top row, lateral views of stage 13 embryos; anterior left, dorsal up. Succeeding rows show third-instar wing, eye-antennal and leg imaginal discs stained to visualize lacZ expression. Grh-lacZ drives nearly ubiquitous expression during (B) late embryogenesis, and (C-E) in imaginal discs. (F-I) The Xvent2-BRE-lacZ reporter is Bmp sensitive. lacZ expression is downregulated at sites of high Dpp signaling. (J-M) Expression of a brklacZ reporter closely matches expression of Xvent2-BRE-lacZ. (N) In situ hybridization showing sites of dpp expression in the embryo. (O-Q) dpp-lacZ expression in imaginal discs. The leading edge of the dorsal ectoderm is marked with a bar in F and J, and an arrow in N; arrowhead in N indicates the boundary between the dorsal and ventral ectoderm. (R) Lysates from S2 cells transfected with the indicated plasmids were used to gel shift the BRE probe. The presence of Mad/Med results in a low mobility complex (band a, lane 2) that is further retarded by anti-FLAG (band b, lane 3) or Shn-Myc (band c, lane 4). The latter complex is supershifted by incubation with anti-Myc (band d, lane 5).