RNA editing by base deamination: more enzymes, more targets, more mysteries

The posttranscriptional modification of messenger RNA precursors (pre-mRNAs ) by base deamination can profoundly alter the physiological function of th e encoded proteins. The recent identification of tRNA-specific adenosine de aminases (ADATs) has led to the suggestion that these enzymes, as well as t he cytidine and adenosine deaminases acting on pre-mRNAs (CDARs and ADARs), belong to a superfamily of RNA-dependent deaminases. This superfamily migh t have evolved from an ancient cytidine deaminase, This article reviews the reactions catalysed by these enzymes and discusses their evolutionary rela tionships.