RNase R is an 3'-->5' exoribonuclease closely related to RNase II, which has been shown to be involved in selective mRNA degradation, particularly of non stop mRNAs in bacteria. RNase R has homologues in many other organisms. When a part of another larger protein has a domain that is very similar to RNase R, this is called an RNase R domain.
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RNase R Wikipedia(Text) CC BY-SA