RNA editing in Drosophila; self-editing, neural target transcripts and evolution. L.P. Keegan¹, A. Leroy¹, G. Ring¹, R. Reenan², M.A. O'Connell¹. 1) Dept Chromosome Biol, MRC Human Genetics Unit, Edinburgh, United Kingdom; 2) University of Connecticut Health Center, 263 Farmington Avenue, Farmington, CT 06030, U.S.A.

   Vertebrate ADAR2 is required to edit the glutamate receptor subunit GluR-B transcript. The function of vertebrate ADAR1 remains unknown. Both ADARs have non-specific RNA editing activity on long RNA duplexes and editing antagonizes transgene silencing in C. elegans. Drosophila has a single CNS-expressed Adar gene with thirty known target transcripts. Adar mutant flies are viable but neurodegenerative and severely defective in walking, flying and mating. We have tested an extensive range of different in vitro-characterized isoforms of Drosophila ADAR as well as human ADARs for rescue in Drosophila, using open field locomotion tests to measure walking activity and RT-PCR analysis to measure editing.
   Drosophila Adar transcripts undergo an ADAR-dependent RNA editing event that changes a conserved serine codon to glycine in the catalytic domain. The edited isoform has drastically reduced editing activity in vitro. An ineditable UAS-Adar cDNA construct is lethal when expressed using a strong act 5C-GAL4 driver but is viable and rescues with other drivers. Self-editing of Adar therefore appears to be mechanism for negative autoregulation and an excess of the genomically encoded form of ADAR is lethal.
   hADAR2 is lethal in Drosophila using the act 5C-GAL4 driver and rescues with other drivers. hADAR1 is not lethal with any driver but also does not rescue. Neurotoxicity due to ADAR2 mutation in vertebrates is mediated by the unedited genome-encoded form of GluR-B. Lethality due to excess ADAR activity in Drosophila may be caused by hyperediting of target transcripts.