To analyze the C-to-U conversion of RNA editing in plant mitochondria, complementary methods are required, which include in vivo, in orgonello, and in vitro approaches. The major obstacle for in vitro assays is the generally observed fragility of the activity in mitochondrial lysates and the corresponding low activity. If seen at all, this activity is often in the range of a few percent conversion of the added templates. We have developed a sensitive assay efficient in such in vitro reactions, which suggests that an RNA helicase may be involved in an effective in vitro RNA editing reaction in plant mitochondria (Takenaka and Brennicke, 2003).