PWWP mutants of the transcriptional cofactor JASPer divert promoter and enhancer activities and suppress position effect variegation in <i>Drosophila</i>

Su(var) genes regulate the formation and maintenance of heterochromatin in Drosophila melanogaster , yet how factors associated with active chromatin influence heterochromatin stability remains poorly understood. Through a new genetic Su(var) screen, we identified two strong suppressors of position effect variegation that remain epistatic to increased Su(var)3-9 dosage in the In(1)w m4h rearrangement. Both mutations affect the PWWP domain of the transcriptional cofactor JASPer , previously implicated in safeguarding transcription within euchromatin. In nuclear cycle (NC14) mutant embryos carrying these mutations, canonical heterochromatic marks (H3K9me2/3) are reduced, while activating histone modifications (H3K4me1 and H3K27ac) accumulate at heterochromatin regions. Whereas wild-type JASPer localizes predominantly to H3K36me3-enriched gene bodies, the mutant protein (JASPer Su140 ) shows reduced chromatin-association and it is redistributed to cis -regulatory elements. Notably, a subset of developmentally regulated enhancers, including the white enhancer, becomes enriched for JASPer Su140 and display elevated enhancer-RNAs transcription at NC14 embryos. Consistent with these molecular changes, mutant embryos exhibit partial decompaction of the heterochromatin domain containing the In(1)w m4h rearrangement and activation of satellite repeat transcription. Together, our findings identify JASPer as a previously unrecognized Su(var) factor and reveal that proper partitioning between euchromatin and heterochromatin domains is critical for maintaining position effect variegation.