Editing-independent effects of Drosophila Adar on heterochromatin silencing

ADAR RNA editing enzymes deaminate selected adenosines to inosines in dsRNA. In Drosophila , inosine in dsRNA inhibits cleavage by Dcr2, and some ADAR proteins contribute an additional, editing-independent inhibition. The Drosophila AdarG isoform, in particular, has been proposed to inhibit HP1-mediated heterochromatin silencing of repetitive sequences initiated by specific dsRNAs. To address the functions of AdarG, we overexpressed it from new UAS-Adar lines, under the control of a temperature-regulated Act5C ts -GAL4 driver. Overexpression of the adult AdarG isoform or catalytically inactive AdarE374A led to larval lethality with some escaper pupae that show an ecdysone-related, head eversion defect. This indicates an editing-independent effect of high Adar expression. Pupae show aberrantly elevated innate immune and early ecdysone gene transcript expression, and no flies eclose. RNAi knockdown of ecdysone receptor A ( EcRA ) or increased expression of the histone H3K9me2,3-associated HP1 protein partially rescues AdarG overexpression defects and normalizes gene expression in rescued progeny flies. In other reports, Drosophila mutants with reduced HP1, or egg (SetDB1), Su(var)3-9 double mutants with reduced histone H3K9me2,3 also produce larvae with ecdysone-related and innate immune defects. We show that overexpressed AdarG inhibits histone H3K9me-mediated epigenetic silencing through an editing-independent effect, most likely at the dsRNA/Dcr2/Ago2 initiation stage.