The short gastrulation shadow enhancer employs dual modes of transcriptional synergy

It remains unclear how a limited amount of maternal transcription factor Dorsal (Dl) directs broad expression of short gastrulation (sog) throughout the presumptive neurogenic ectoderm in the Drosophila early embryo. Here, we present evidence that the sog shadow enhancer employs dual modes of transcriptional synergy to produce this broad pattern. Bioinformatics analyses indicated that a minimal enhancer region, systematically mapped in vivo, contains five Dl-, three Zelda (Zld)-, and three Bicoid (Bcd)-binding sites; four of these five Dl-binding sites are closed linked to two Zld- and two Bcd-binding sites. Mutations of either the linked Zld- or Bcd-binding sites led to severe reduction in lacZ expression width, length, and/or strength in transgenic embryos. In addition, alteration of the helical phasing in this enhancer region by insertion of spacer sequences between linked sites also resulted in aberrant lacZ expression. These results suggest that synergistic interactions between Dl and Zld and between DI and Bcd are required for broad sog expression.