Résumé
The polyhomeotic (ph) gene of Drosophila is a member of the Polycomb group of genes and encodes a chromatin protein required for negative regulation of homeotic genes and other loci, in particular the ph locus itself. We have studied the genetic control of ph transcription during development. Early ph expression is under the control of bicoid and engrailed as activators and of oskar as an inhibitor. The negative autoregulation of ph starts at the blastoderm stage and is partly mediated by a transvection effect. As the number of functional copies of ph increases in the same genome, a concomitant reduction of the transcription of each copy is observed. This regulation is ensured, likely at the chromatin level, positively by the trithorax group and negatively by the Polycomb group gene products like a homeotic gene, but it occurs in the same cells. We propose that an equilibrium between these two states of chromatin activity ensures an accurate level of ph transcription.