Repression and activation of transcription by Engrailed: Involvement of Extradenticle, Ubx and Abda as cofactors. C. ALEXANDRE , J.P. VINCENT. Mammalian Development, NIMR, London, UK.

   Engrailed is the archetypal repressor. Indeed, the repressor domain of Engrailed confers repressive activity to a variety of heterologous proteins. Nevertheless, genetic evidence suggests that Engrailed is both an activator and a repressor of transcription. We find that VP16-Engrailed (where the repression domain of Engrailed is replaced with the strong activation domain of VP16) represses genes that are normally activated by Engrailed (such as hedgehog and engrailed itself). This suggests that, in the wildtype, repression by Engrailed is indirect and involves the repression of a repressor. We identify this repressor as the Zn-finger proteins encoded by sloppy-paired 1 and 2 (Engrailed represses sloppy-paired expression and Sloppy-paired represses the two positive targets of Engrailed). However, repression of sloppy-paired is not sufficient to activate Engrailed targets. True (maybe direct) activation by Engrailed occurs in parallel. True activation requires Extradenticle, a cofactor involved in the activity of a variety of homeodomain-containing proteins. We find that activation of target genes by Engrailed-VP16 also require extradenticle. This and other evidence lead us to suggest that Extradenticle helps Engrailed in target recognition (and does not really convert Engrailed from a repressor to an activator). Consistent with this view, in the thorax, Extradenticle is also required for repression by Engrailed (of genes such as sloppy-paired and cubitus interruptus). In the abdomen, Extradenticle is not required for repression but his role is fulfilled by the homeotic genes Ubx and ABDA. In the absence of these two genes and extradenticle, overexpressed Engrailed is no longer capable of repressing genes. We suggest that, like Extradenticle, Ubx and Abda could be cofactor of Engrailed. We are currently assessing this suggestion further by testing whether Ubx and Abda interact directly with Engrailed.