Although some viroid-like satellite RNAs possess self-cleavage and self-ligation 
activities, we show that the peach latent mosaic viroid (PLMVd) is unique among 
all known viroids since it also has such activities. These catalytic activities 
should have important roles in the rolling circle replication of PLMVd. 
According to this proposed mechanism, self-cleavage of the multimeric strands 
occurs via hammerhead structures producing monomers possessing 2',3'-cyclic 
phosphate and 5'-hydroxyl termini. In the most stable predicted secondary 
structure for PLMVd these two extremities are juxtaposed, in order for 
self-ligation to occur. To establish the nature of the phosphodiester bond 
produced by self-ligation, we followed the classical procedure of complete 
enzymatic RNA hydrolysis coupled with thin layer chromatography fractionation. 
Using this procedure, we report that the self-ligation of PLMVd transcripts 
produces almost exclusively the 2',5' isomer (>96%). Primer extension assays 
also revealed that reverse transcriptase can read througth this 2', 5' linkage, 
suggesting that it does not prevent further replication of the viroid. Moreover, 
we have observed that this 2',5' linkage is resistant to the debranching 
activity contained in nuclear extracts, as well as being capable of preventing 
further viroid self-cleavage. Thus, if viroids do indeed self-ligate in vivo, 
the resulting 2', 5'-phosphodiester bond could contribute to the stability of 
these RNA species. Finally, an analysis of both the sequence and the structural 
requirements for hammerhead self-cleavage and self-ligation suggests that these 
two RNA processes may be interrelated. We hypothesize that the intramolecular 
self-ligation which produces circular conformers may contribute to the 
circularization step of the rolling circle replication, while the intermolecular 
non-enzymatic ligation is a potential mechanism for the sequence reassortment of 
viroids and viroid-like species.