A-to-I editing challenger or ally to the microRNA process.

Adenosine to inosine (A-to-I) modification by the ADAR (adenosine deaminase that acts on RNA) enzymes perform the most common type of RNA editing in metazoans. ADARs use double stranded RNA as substrates but allow interruptions of bulges and loops in the structure. It is well known that these enzymes can use messenger RNA as targets for A-to-I editing and thereby recode the transcript. Both ADAR1 and ADAR2 have been proven to be able to also target short double stranded RNA molecules of the same size as a microRNA. However, it is not until recently shown that A-to-I editing occurs in microRNAs and its precursors. Since the editing activity is found both in the nucleus and the cytoplasm there are several steps during the microRNA maturation pathway that can be targeted for modification. This review will give an overview of what is known today about the interactions between the endogenous RNA interference process and RNA editing. It will also give some insight into the power of A-to-I modification in its ability to increase the variety of microRNA gene silencing.