Résumé
Polycomb group (PcG) and trithorax group (trxG) gene products are responsible for the maintenance of repressed and active expression patterns of many developmentally important regulatory genes including the homeotic genes. In
Drosophila embryos, Polycomb protein and the trxG protein GAGA factor colocalize at the
Fab-7 DNA element of the bithorax complex. In transgenic lines, the
Fab-7 element induces extensive silencing on a flanking GAL4-driven
lacZ reporter and mini-
white genes. However, a short single pulse of GAL4 during embryogenesis is sufficient to release PcG-dependent silencing from the transgene. Such an activated state of
Fab-7 is mitotically inheritable through development and can be transmitted in a GAL4-independent manner to the subsequent generations through female meiosis. Thus,
Fab-7 is a switchable chromosomal element, which can convey memory of epigenetically determined active and repressed chromatin states.