RNA editing enzymes in the bdeloid rotifer Adineta vaga.

Small freshwater invertebrates, rotifers of class Bdeloidea, are attractive to researchers because of their long-term asexuality and ability to survive repeated desiccation and extremely high doses of ionizing radiation. Recently, whole-genome sequencing of the bdeloid rotifer Adineta vaga by the international A. vaga Sequencing Consortium provided new insights into rotifer biology. We found that A. vaga has a highly efficient mitochondrial RNA editing system, associated with dramatic expansion of cytidine deaminases. RNA editing is a post-transctiptional process, which results in alteration of RNA nucleotide sequence, and causes protein diversification. There are several types of RNA editing, such as cytidine-to-uridine (C-to-U) editing, which may create new start or stop codons; and uridine-to-cytidine and adenosine-to-inosine (A-to-I) editing, which may cause alterations in splice sites. The most common is C-to-U and A-to-I editing, resulting from RNA deamination by enzymes called deaminases. The A. vaga genome contains 134 regions of homology to cytidine deaminases responsible for C-to-U editing, 88 of which are annotated as proteins. Notably, two of these proteins do not have an N-terminal domain responsible for mitochondrial targeting, therefore being prime candidates for editing of non-mitochondrial RNAs, such as small RNAs. Two ADAR-like deaminases responsible for A-to-I editing are also present. The abundance of cytidine deaminases correlates well with the existence of seven mitochondrial RNA sites at which C-to-T editing occurs with nearly 100% efficiency. Such editing was previously shown only for plants and a protist, and therefore represents the first case of mtRNA editing in metazoans.