Bicoid functions without its TATA-binding protein-associated factor interaction domains

Four maternal systems are known to pattern the early Drosophila embryo. The key component of the anterior system is the homeodomain protein Bicoid (Bc d). Bcd needs the contribution of another anterior morphogen, Hunchback (Hb ), to function properly: Bcd and Hb synergize to organize anterior developm ent. A molecular mechanism for this synergy has been proposed to involve sp ecific interactions of Bcd and Hb with TATA-binding protein-associated fact ors (TAF(II)s) that are components of the general transcription machinery. Bcd contains three putative activation domains: a glutamine-rich region, wh ich interacts in vitro with TAF(II)110; an alanine-rich domain, which targe ts TAF(II)60; and a C-terminal acidic region, which has an unknown role. We have generated flies carrying bcd transgenes lacking one or several of the se domains to test their function in vivo. Surprisingly, a bcd transgene th at lacks all three putative activation domains is able to rescue the bcd(E1 ) null phenotype to viability. Moreover, the development of these embryos i s not affected by the presence of dominant negative mutations in TAF(II)110 or TAF(II)60. This means that the interactions observed in vitro between B cd and TAF(II)60 or TAF(II)110 aid transcriptional activation but are dispe nsable for normal development.