Figure 5.

A model for the regulation of L71 late gene transcription by E74 and the BR-C. The timing of BR-C Z1, E74B, and E74A expression in late larval salivary glands is represented by boxes. Low titer ecdysone pulses in early and mid-third instar larvae induce the E74B protein and BR-C Z1 isoform, respectively (18–20). Z1 acts as a general activator of L71 gene transcription, but is prevented from inducing L71 promoter activity during larval development by the presence of E74B. Just before puparium formation, the high titer late larval ecdysone pulse represses E74B and induces E74A. The presence of both Z1 and E74A proteins in newly formed prepupae contributes to the coordination of late gene induction. Our studies also show that one or more other repressors and/or inducers must contribute to this regulatory circuit.