Fig. 1. The E(spl)m PNC enhancer drives strong expression in non-SOPs but is directly repressed by Su(H) in SOPs. (A) Diagram of reporter constructs in which wild-type and mutant versions of the E(spl)m PNC cis-regulatory module drive expression of GFP or RFP via a minimal Hsp70 promoter. Positions of the five Su(H)-binding sites (S) and single proneural (PN) protein-binding site (E box) are indicated; mutant sites are indicated by a red cross. (1) Wild-type module; (2) module with E box mutated (Em); (3) Su(H)-binding sites mutated (Sm); (4) E box and Su(H) sites mutated (EmSm). (B-D,F-H) Single wing disc from a late third-instar larva carrying one copy each of E(spl)m-RFP and E(spl)m Sm-GFP and stained with anti-Hindsight (Hnt) antibody (C) to mark SOPs. Insets show higher-magnification views of the boxed region of the wing disc. (B'-D',F'-H') Microchaete row (adjacent to the dorsal midline) on the notum of a single pupa of the same genotype 14 hours after puparium formation (APF), also stained with anti-Hnt (C'). (B,B') RFP signal driven by the wild-type module is strong in most non-SOP cells but minimal or absent in SOPs (B,D,B',D'). (E) Mutation of the E box (Em) causes severe loss of PNC expression; expression is retained at the wing margin and in a small subset of PNC cells. (F,G,F',G') Mutation of the Su(H)-binding sites (Sm) abolishes or severely lowers reporter expression in non-SOP cells of the PNCs but causes strong ectopic expression in SOPs. (H,H') Overlay of B (B') and F (F') highlights dramatic change in the pattern of reporter gene activity when Su(H) sites are mutated. (I) Mutation of both the E box and the Su(H) sites (EmSm) demonstrates that ectopic SOP expression of the Sm mutant (F) is dependent on direct input from the proneural proteins.