ADAR2 A®I editing: site selectivity and editing ef®ciency are separate events

ADAR enzymes, adenosine deaminases that act on RNA, form a family of RNA editing enzymes that convert adenosine to inosine within RNA that is completely or largely double-stranded. Site-selective A®I editing has been detected at speci®c sites within a few structured pre-mRNAs of meta-zoans. We have analyzed the editing selectivity of ADAR enzymes and have chosen to study the natur-ally edited R/G site in the pre-mRNA of the gluta-mate receptor subunit B (GluR-B). A comparison of editing by ADAR1 and ADAR2 revealed differences in the speci®city of editing. Our results show that ADAR2 selectively edits the R/G site, while ADAR1 edits more promiscuously at several other adeno-sines in the double-stranded stem. To further under-stand the mechanism of selective ADAR2 editing we have investigated the importance of internal loops in the RNA substrate. We have found that the imme-diate structure surrounding the editing site is important. A purine opposite to the editing site has a negative effect on both selectivity and ef®ciency of editing. More distant internal loops in the sub-strate were found to have minor effects on site selectivity, while ef®ciency of editing was found to be in¯uenced. Finally, changes in the RNA structure that affected editing did not alter the binding abilities of ADAR2. Overall these ®ndings suggest that binding and catalysis are independent events