Dual functionality of cis-regulatory elements as developmental enhancers and Polycomb response elements.
Genes Dev, 31(6):590-602.
Developmental gene expression is tightly regulated through enhancer elements, which initiate dynamic spatio-temporal expression, and Polycomb response elements (PREs), which maintain stable gene silencing. These two cis-regulatory functionsare thought to operate through distinct dedicated elements. By examining the occupancy of the Drosophila pleiohomeotic repressive complex (PhoRC) during embryogenesis, we revealed extensive co-occupancy at developmental enhancers. Using an established in vivo assay for PRE activity, we demonstrated that a subset of characterized developmental enhancers can function as PREs, silencing transcription in a Polycomb-dependent manner. Conversely, some classic Drosophila PREs can function as developmental enhancers in vivo, activating spatio-temporalexpression. This study therefore uncovers elements with dual function: activating transcription in some cells (enhancers) while stably maintaining transcriptionalsilencing in others (PREs). Given that enhancers initiate spatio-temporal gene expression, reuse of the same elements by the Polycomb group (PcG) system may help fine-tune gene expression and ensure the timely maintenance of cell identities.