Résumé
The
polyhomeotic (ph) gene is a member of the
Polycomb group of genes (
Pc-G), which are required for the maintenance of the spatial expression pattern of homeotic genes. In contrast to homeotic genes,
ph is ubiquitously expressed and it is quantitatively regulated.
ph is negatively regulated by the
Pc-G genes, except
Psc, and positively regulated by the antagonist
trithorax group of genes (
trx-G), suggesting that
Pc-G and
trx-G response elements (PREs and TREs) exist at the
ph locus. In this study, we have functionally characterized PREs and TREs at the
ph locus that function in transgenic constructs. We have identified a strong PRE and TRE in the
ph proximal unit as well as a weak one in the
ph distal unit. The PRE/TRE of both
ph units appear atypical compared with the well-defined homeotic maintenance elements because the minimal
ph proximal response element activity requires at least 2 kb of sequence and does not work at long range. We have used chromatin immunoprecipitation experiments on cultured cells and embryos to show that
Pc-G proteins are located in restricted regions, close to the
ph promoters that overlap functionally defined PRE/TREs. Our data suggest that
ph PRE/TREs are
cis-acting DNA elements that modulate rather than silence
Pc-G- and
trx-G-mediated regulation, enlarging the role of these two groups of genes in transcriptional regulation.