The properties of a tRNA-specific adenosine deaminase from Drosophila melanogaster support an evolutionary link between pre-mRNA editing and tRNA modification

Pre-mRNA editing involving the conversion of adenosine to inosine is mediat ed by adenosine deaminases that act on RNA (ADAR1 and ADAR2). ADARs contain multiple double-stranded RNA(dsRNA)-binding domains in addition to an aden osine deaminase domain. An adenosine deaminase acting on tRNAs, scTad1p (al so known as scADAT1), cloned from Saccharomyces cerevisiae has a deaminase domain related to the ADARs but lacks dsRNA-binding domains. We have identi fied a gene homologous to scADAT1 in the region of Drosophila melanogastert Adh chromosome II. Recombinant Drosophila ADAT1 (dADAT1) has been expresse d in the yeast Pichia pastoris and purified. The enzyme has no activity on dsRNA substrates but is a tRNA deaminase with specificity for adenosine 37 of insect alanine tRNA. dADAT1 shows greater similarity to vertebrate ADARs than to yeast Tad1p, supporting the hypothesis of a common evolutionary or igin for ADARs and ADATs. dAdat1 transcripts are maternally supplied in the egg. Zygotic expression is widespread initially and later concentrates in the central nervous system.