DROSOPHILA GOOSECOID REQUIRES A CONSERVED HEPTAPEPTIDE FOR REPRESSIONOF PAIRED-CLASS HOMEOPROTEIN ACTIVATORS

Goosecoid (Gsc) is a homeodomain protein expressed in the organizer re gion of vertebrate embryos. Its Drosophila homologue, D-Gsc, has been implicated in the formation of the Stomatogastric Nervous System. Alth ough there are no apparent similarities between the phenotypes of muta tions in the gsc gene in flies and mice, all known Gsc proteins can re scue dorsoanterior structures in ventralized Xenopus embryos. We descr ibe how D-Gsc behaves as a transcriptional repressor in Drosophila cel ls, acting through specific palindromic HD binding sites (P3K). D-Gsc is a 'passive repressor' of activator homeoproteins binding to the sam e sites and an 'active repressor' of activators binding to distinct si tes. In addition, D-Gsc is able to strongly repress transcription acti vated by Paired-class homeoproteins through P3K, via specific protein- protein interactions in what we define as 'interactive repression'. Th is form of repression requires the short conserved GEH/eh-1 domain, al so present in the Engrailed repressor. Although the GEH/eh-1 domain is necessary for rescue of UV-ventralized Xenopus embryos, it is dispens able for ectopic induction of Xlim-1 expression, demonstrating that th is domain is not required for all Gsc functions in vivo. Interactive r epression may represent specific interactions among Prd-class homeopro teins, several of which act early during development of invertebrate a nd vertebrate embryos.