NEITHER THE HOMEODOMAIN NOR THE ACTIVATION DOMAIN OF BICOID IS SPECIFICALLY REQUIRED FOR ITS DOWN-REGULATION BY THE TORSO RECEPTOR TYROSINEKINASE CASCADE

Bicoid (Bcd) is a maternal morphogen responsible for patterning the he ad and thorax of the Drosophila embryo. Correct specification of head structure, however, requires the activity of the Torso receptor tyrosi ne kinase cascade, which also represses expression of Bcd targets at t he most anterior tip of the embryo. Here, we investigate the role of b oth the homeodomain (HD) and the activation domain of Bcd in the anter ior repression of its targets. When a Bcd mutant protein whose HD has been replaced by the Gal4 DNA-binding domain is expressed in early emb ryos, a reporter gene driven by Gal4 DNA-binding sites is first activa ted in an anterior domain and then repressed from the anterior pole. T he down-regulation of Bcd-Gal4 activity requires torso function but do es not depend on endogenous bcd activity, indicating that the Bcd prot ein alone and none of its targets is required to mediate the effect of torso. Functional analysis of a chimeric protein, whose activation do main has been replaced by a generic activation domain, indicates that the activation domain of Bcd is also not specifically required for its down-regulation by Torso. We propose that Torso does not affect the a bility of Bcd to bind DNA, but instead directs modification of Bcd or of a potential Bcd co-factor, which renders the Bcd protein unable to activate transcription.