Promoter competition and Polycomb response elements govern transvection efficiency between co-regulated <i>engrailed</i> and <i>invected</i> genes in <i>Drosophila</i>

Transvection is a phenomenon where gene regulatory elements interact between different chromosomes, adding an additional layer of regulatory control beyond traditional cis-interactions. Although transvection effects have been characterized for many individual genes in Drosophila, it remains unclear how trans-interactions occur among multiple co-regulated genes where enhancers are shared. Here we demonstrate that transvection is supported at the engrailed-invected (en-inv) locus, where transcription of the two developmental genes is coordinated by common enhancers. Our data show that the presence of the en promoter in cis to the enhancers prevents trans-activation of inv, but removal of this promoter enables robust transvection, demonstrating competition between heterologous promoters in trans. We also find that local Polycomb response elements (PREs) enhance transvection reliability but are not strictly required for trans-activation. Furthermore, our analysis reveals that transvection at this locus is developmentally regulated, occurring efficiently in third instar larval tissues and late-stage embryos but not in early embryonic stages. Finally, we show that en-inv transvection can be reconstituted using transgenic constructs at an ectopic chromosomal location, where it produces a striking reciprocal expression pattern between en in cis and inv in trans, suggesting that these enhancers can choose to activate one promoter or the other in a stochastic manner.