Contributions to gene activation by multiple functions of Bicoid

Bicoid is a Drosophila morphogenetic protein required for the development o f anterior structures in the embryo. To gain a better understanding of how Bicoid works as a transcriptional activator, we systematically analysed var ious functions of Bicoid required for gene activation. We provide evidence suggesting that Bicoid is an intrinsically weak activator. First, our bioch emical experiments demonstrate that the Bicoid-DNA complexes are very unsta ble, suggesting a weak DNA-binding function of Bicoid. This idea is further supported by our experiments demonstrating that the same number of LexA-Bi coid fusion molecules can activate transcription more effectively from LexA sites than from Bicoid sites. Secondly, we demonstrate that transcriptiona l activation by the weak activator Bicoid is readily influenced by. the loc al enhancer environment. These influences are decreased when the Bicoid fun ction is enforced by attaching to it either a known dimerization domain or the strong activation domain VP16. VP16 can also compensate for the loss of some Bicoid sites in an enhancer element. Our experiments demonstrate that the outcome of transcriptional activation by Bicoid is determined by multi ple weak functions that are interconnected, a finding that can further help us to understand how this morphogenetic protein achieves its molecular fun ctions.