Fig. 8. Model for cell-type-specific transcription of pros. At the top is a schematic of the pros mini-enhancer arrayed with the different transcription factors on their binding sites. Several factors act downstream of Notch and Egfr signals. Notch acting through Su(H) inhibits production of the Svp repressor (red). Svp might repress the enhancer directly, or it might function through an intermediary. Egfr signaling inhibits the Yan repressor (red) and stimulates Pnt (green). Several factors work cooperatively to increase the transcription activity of the enhancer, and these are colored yellow. They are Lz, Gl and So/Eya. At the bottom is a model in which the enhancer becomes competent prior to its activation of pros transcription. (1) The inactive enhancer is in a repressive chromatin environment when neither Notch nor Egfr signals are received. (2) It transits towards competence when either Egfr or Notch alone is active. (3) It is competent when both Notch and Egfr signals are received by a cell. Competence may be expressed as a relaxation of chromatin structure, or the ability of the enhancer to connect to the promoter and facilitate transcription, or both. The formation of a cooperative enhancer complex (yellow ovals) composed of Lz, Gl and So/Eya is required for the enhancer to set the level of transcription. Competence may enable the cooperative enhancer complex to form (left). Alternatively, the enhancer complex may form on a non-competent enhancer but may only connect with the promoter when competence is achieved (right).