Structural and Ionic Requirements for Self-cleavage of Virusoid RNAs and trans Self-cleavage of Viroid RNA

stranded plant pathogenic RNAs can self-cleave at specific sites in vitro in the presence of Mg ++ (Buzayan et al. 1986a; Hutchins et al. 1986; Prody et al. 1986; Forster and Symons 1987a). This property is considered to be important in their replication in vivo by a rolling-circle mechanism. All of these self-cleavage reactions generate 5'-OH and 2',3'-cyclic phosphodiester termini in what is essentially a transphosphorylation reaction involving attack by the free T-OH group on the 3 '5 '-phosphodiester linkage (Fig. 1A). These self-cleavage reactions are clearly different from the self-splicing reactions involved in the removal of introns from rRNA and mRNA precursors that produce excised fragments with 5'-phosphate and 3'-OH termini (Cech and Bass 1986; Sharp 1987).