POLYHOMEOTIC APPEARS TO BE A TARGET OF ENGRAILED REGULATION IN DROSOPHILA

In Drosophila, Engrailed is a nuclear regulatory protein with essentia l roles in embryonic segmentation and in normal development of posteri or compartments. One of its regulatory targets appears to be polyhomeo tic (ph), a Polycomb group gene. We observed, by immunostaining, that Engrailed protein binds to the site of the polyhomeotic locus in regio n 2D of polytene chromosomes. The same analysis carried out on a trans genic line containing one copy of a P(ph-lacZ) construct shows an addi tional Engrailed-binding site at the location of the insert. In vivo, polyhomeotic depends on engrailed function in germ-band-elongated embr yos, when engrailed and polyhomeotic genes are expressed in similar pa tterns. By in vitro immunoprecipitations and gel shift assays, we iden tified two classes of high affinity Engrailed-binding sites upstream o f each of the two polyhomeotic transcription units. DNA fragments cont aining these sites were also immunoprecipitated from embryonic UV cros slinked chromatin in presence of anti-Engrailed antibody. These result s suggest that polyhomeotic activation in germ-band-elongated embryos could be mediated by Engrailed-binding to these sites.