Ribonuclease L (RNase L) is a key enzyme in the 2-5A host defense system, and its activity is strictly regulated by an unusual 2¢,5¢-linked oligoadenylate (2-5A).A bipartite model, in which the N-terminal half of RNase L is responsible for the 2-5A binding and the C-terminal half alone is able to hydrolyse the substrate RNA, has been proposed on the basis of the results of deletion mutant analyses [Dong, B.& Silverman, .