DOWN-REGULATION OF THE DROSOPHILA MORPHOGEN BICOID BY THE TORSO RECEPTOR-MEDIATED SIGNAL-TRANSDUCTION CASCADE

Anterior body pattern in Drosophila is specified by the graded distrib ution of the bicoid protein (bcd), which activates subordinate genes i n distinct anterior domains. Subsequently, transcription of these targ et genes is repressed at the anterior pole owing to the activity of th e receptor tyrosine kinase torso (tor). We show that both activation b y bcd and repression by tor can be reproduced by a minimal promoter co ntaining only bcd-binding sites upstream of a naive transcriptional st art site. Repression requires the D-raf kinase and is associated with phosphorylation of bcd protein. Repression does not require either tai lless or huckebein, which were previously thought to constitute the so le zygotic output of the tor signaling system. Finally, addition of a heterologous transcriptional activation domain to bcd renders the prot ein insensitive to tor-mediated repression. We propose that phosphoryl ation resulting from the activity of the tor signal transduction casca de down-regulates transcriptional activation by the bcd morphogen.