Figure 1

RNA polymerase II (Pol II) transcription through the bxd PRE/TRE leads to the generation of noncoding tre1, tre2 and tre3 RNAs. These RNAs remain tethered to their site of origin, presumably via base pairing with the unwound single-stranded DNA. The histone methyltransferase ASH1 specifically interacts with tre1, tre2 and tre3 RNAs and thus becomes targeted to the PRE/TRE. Through its SET domain, ASH1 catalyzes the trimethylation (Me) of lysines 4 and 9 on histone H3, and of lysine 20 on histone H4. This leads to the activation of the chromatin structure and, as a consequence, the stimulation of Ubx expression.