Massive RNA editing in ascetosporean mitochondria

Ascetosporeans are parasitic protists of invertebrates. As only two species of Mikrocytida, an ascetosporean subgroup, have ever been sequenced deeply and analyzed using cells isolated from infected organisms, it was shown that their mitochondria are functionally reduced and the organellar genome is lacking. However, molecular studies on other ascetosporeans have not been conducted, and whether reduced mitochondria is common in ascetosporeans remains unclear. In the present study, we established two cultures of Paradinida, another ascetosporean subgroup, and reconstructed their mitochondrial genomes. As they were compared with their RNA-seq data, massive A-to-I and C-to-U types of RNA editing were detected. Many editing sites are shared between two paradinids, but strain-unique sites also exist. As the mitochondrial genes are involved in the electron transfer system, their mitochondria are not functionally reduced, unlike that in Mikrocytida. Furthermore, we detected adenosine deaminase acting on RNA (ADAR), which is a key enzyme of A-to-I substitution, in paradinids as well as several other protists. Immunostaining showed that this ADAR is specifically localized in the mitochondria of paradinids, suggesting that A-to-I substitution in paradinid mitochondria is mediated by ADAR. These findings elucidated the functional diversity and evolutionary process of ascetosporean mitochondria as well as ADAR.