Context-dependent transcriptional regulation by Drosophila Polycomb Response Elements
Polycomb Response Elements (PREs) are cis-acting DNA sequences that confer heritable states of Drosophila HOX gene expression by anchoring Polycomb and Trithorax Group (PcG and TrxG) chromatin modifiers. PREs are also associated with hundreds of other Drosophila genes, most of which are regulated dynamically in response to developmental and physiological context, rather than heritably like HOX genes. Here, we assess the role(s) PREs play at these other loci by analyzing how genomic inserts of a transgenic form of the HOX gene Ultrabithorax (Ubx) can both control and respond to neighboring genes depending on the presence of a single, excisable PRE. Our results support the view that PREs and their associated PcG and TrxG modifiers act primarily to confer quantitative, rather than qualitative, influences on gene expression with the response of any given gene depending on how it integrates this information with other regulatory elements in the local genomic milieu. They also show that PREs can act on neighboring genes selectively and at remarkably long range, but that any given gene can be susceptible or impervious to PRE/PcG/TrxG input depending on context. Finally, we find that transcription and PRE/PcG-dependent silencing are not mutually exclusive: a Ubx transgene inside the intron of a continuously transcribed "host" gene is nevertheless silenced by its resident PRE. We posit that the widely accepted roles of PcG and TrxG complexes in maintaining heritable states of gene expression apply only to a limited coterie of target genes such as HOX genes that are evolutionarily selected to exclude regulatory elements that can over-ride this control.