Function of Da-N subdomains in mediating NTC-bHLH A transcriptional synergy. A, diagram of Daughterless proteins used in this study. The basic-helix-loop-helix (bHLH) DNA binding domain (DBD) and "loop-helix" (LH) transcription activation domain are indicated. B, transcription assays with the native m8 promoter (m8-WT) and co-expression of NICD and Daughterless N-terminal protein domains. Asterisks in expts. 6 and 7 indicate promoter activity significantly greater relative to either expt. 2, 3 or 4. C, either deletion or replacement of the Da-N1 subdomain blocks NTC-bHLH A transcriptional synergy on the m8-WT promoter in S2 cells. Asterisk in expt. 3 indicates promoter activity significantly greater than observed in expt. 2. D, transcription activation potential of the Da-WT, Da-ΔN1 and VP16-DabHLH proteins on the artificial 4A promoter. The single asterisk in expt. 3 indicates promoter activity that is significantly greater than the basal promoter activity of the reporter plasmid (expt. 1), but less than the promoter activity observed with the co-expression of Ac/Da (expt. 2). The double asterisk in expt. 5 indicates promoter activity significantly greater than the basal promoter activity of the reporter plasmid (expt. 1), but not significantly different than the promoter activity observed with the co-expression of Ac/Da (expt. 2). For all panels, differences with p < 0.01 were considered significant.