The Drosophila Engrailed homeoprotein has been shown to activate direc
tly a Polycomb-group gene, polyhomeotic, during embryogenesis. The mol
ecular mechanism involved in this activation has been studied. Two dif
ferent types of Engrailed-binding fragments have been detected within
the polyhomeotic locus. The P1 and D1 fragments contain several 'TTAAT
TGCAT' motifs, whereas the D2 fragment contains a long 'TAAT' stretch
to which multiple copies of Engrailed bind cooperatively. Another home
odomain-containing protein, Extradenticle, establishes protein-protein
interactions with Engrailed on the D2 fragment, We have shown by CAT
assays that both types of Engrailed-binding sites (P1 or D1 and D2), a
s well as Extradenticle, are necessary to obtain activation by Engrail
ed. In vivo, we have also shown that normal polyhomeotic expression de
pends on extradenticle expression, Moreover, in the absence of Extrade
nticle, overexpression of Engrailed protein represses polyhomeotic exp
ression.