RNA Editing

RNA editing has been observed in mRNA, tRNA and rRNA. It has been detected in mitochondria and chloroplasts and in nuclear encoded RNAs but as yet not in procaryotes. RNA editing can be divided into two categories. In one, called insertion/deletion RNA editing, nucleotides are inserted or deleted. The best known example occurs in mitochondrial mRNAs of trypanosomes, a kind of protozoan. In this case, a number of U nucleotides are inserted or deleted to create the translatable mRNAs. Specific small RNAs, guide RNAs interact with the mRNA to define the position of editing.

	In the second category of RNA editing, specific nucleotides are modified to change one nucleotide into another. One example is the de-amination of cytidine which occurs in mammalian apolipo protein B mRNA in the intestine. Here, a specific C is changed into a U, introducing a stop codon for translation and thereby a shorter version of the protein. A second example is de-amination of adenosine to inosine. Inosine is read as guanosine by the translation machinery. The adenosine is present in a double stranded region of the mRNA and the enzyme adenosine de-aminase that acts on RNA, ADAR, catalyzes the reaction. Examples of mRNAs edited by ADAR are mRNA encoding glutamate receptors, serotonin receptors and potassium channels in the central nervous system and hepatitis delta virus RNA.