Double-stranded RNA adenosine deaminases ADAR1 and ADAR2 have overlapping specificities

Adenosine deaminases that act on RNA (ADARs) deaminate adenosines to produc e inosines within RNAs that are largely double-stranded (ds). Like most dsR NA binding proteins, the enzymes will bind to any dsRNA without apparent se quence specificity. However, once bound, ADARs deaminate certain adenosines more efficiently than others. Most of what is known about the intrinsic de amination specificity of ADARs derives from analyses of Xenopus ADAR1. In a ddition to ADAR1, mammalian cells have a second ADAR, named ADAR2; the deam ination specificity of this enzyme has not been rigorously studied. Here we directly compare the specificity of human ADAR1 and ADAR2. We find that, l ike ADAR1, ADAR2 has a 5' neighbor preference (A approximate to U > C = G), but, unlike ADAR1, also has a 3' neighbor preference (U = G > C = A). Simu ltaneous analysis of both neighbor preferences reveals that ADAR2 prefers c ertain trinucleotide sequences (U (A) under barU, A (A) under barG, U (A) u nder barG, A (A) under barU). In addition to characterizing ADAR2 preferenc es, we analyzed the fraction of adenosines deaminated in a given RNA at com plete reaction, or the enzyme's selectivity. We find that ADAR1 and ADAR2 d eaminate a given RNA with the same selectivity, and this appears to be dict ated by features of the RNA substrate. Finally, we observed that Xenopus an d human ADAR1 deaminate the same adenosines on all RNAs tested, emphasizing the similarity of ADAR1 in these two species. Our data add substantially t o the understanding of ADAR2 specificity, and aid in efforts to predict whi ch ADAR deaminates a given editing site adenosine in vivo.