INTRAMOLECULAR AND INTERMOLECULAR NONENZYMATIC LIGATIONS OCCUR WITHINTRANSCRIPTS DERIVED FROM THE PEACH LATENT MOSAIC VIROID

We report here the nonenzymatic self-ligation of transcripts correspon ding to the peach latent mosaic viroid (PLMVd). This is the first desc ription of this process with viroid sequences, although it has been re ported to occur with human hepatitis delta virus RNA. Self-ligation oc curs when the 5'-hydroxyl and the 2',3'-cyclic phosphate termini produ ced by the hammerhead self-cleavage of the viroid RNA are juxtaposed b y the viroid rod-like structure, and a phosphodiester bond is formed b etween the two following hydrolysis of the cyclic phosphate. Unit-leng th transcripts undergo intramolecular folding, and their subsequent se lf-ligation produces circular molecules. The self-ligation observed in vitro may contribute to PLMVd circularization during rolling circle r eplication; however, this does not exclude the possibility that a host RNA ligase catalyzes the ligation steps in vivo. Like self-cleavage, self-ligation is probably an ancestral reaction, and the enzyme-cataly zed ligation most likely evolved from this primitive mechanism. Furthe rmore, the intermolecular self-ligation of annealed transcripts derive d from PLMVd is demonstrated, suggesting a possible mechanism for sequ ence reassortment in viroids. (C) 1995 Academic Press, Inc.