ENY2 Protein Is Present at the Histone Locus Body HLB As Part of Complexes with Different Functions in Transcription and mRNA Export

The evolutionarily conserved transcription factor ENY2 is a component of several distinct complexes that regulate various stages of gene expression: the SAGA transcription coactivator, the THO transcription elongation complex involved in mRNA processing, and the TREX-2 complex responsible for mRNA export from the nucleus to the cytoplasm. Previous works in D. melanogaster have shown that SAGA associates with the histone locus body (HLB) biomolecular condensate, where histone mRNA transcription and processing take place, whereas TREX-2 interacts with histone mRNA and is involved in its export from the nucleus to the cytoplasm. This study demonstrates that ENY2 is a component of the HLB. ENY2 is present at the histone locus on D. melanogaster polytene chromosomes. ENY2 associates with the chromatin of the histone gene cluster as part of the SAGA coactivator complex and the THO transcription elongation complex, which also interacts with histone gene chromatin. ENY2 and subunits of the SAGA, THO, and TREX-2 complexes interact with FLASH, a specific constituent structural component of the HLB. Thus, ENY2 is present at the HLB biomolecular condensate and participates in histone gene expression as part of three functionally diverse complexes: SAGA, THO, and TREX-2.