A 451-NUCLEOTIDE CIRCULAR RNA FROM CHERRY WITH HAMMERHEAD RIBOZYMES IN ITS STRANDS OF BOTH POLARITIES

The sequence of 451 nucleotides of a cherry small circular RNA (csc RN A1) associated with a cherry disease has been determined, Both csc RNA 1 and its complementary strand can form hammerhead structures similar to those found previously in other plant and animal small RNAs. In the branched secondary structure of lo,vest free energy of csc RNA1, the sequences involved in the hammerhead structures, which comprise approx imately one-fourth of this RNA, are found opposite each other, forming part of a rod-like segment, Plus-and minus-strand full-length transcr ipts of csc RNA1 self-cleaved during transcription and after purificat ion, as predicted by the hammerhead structures, which are stable and v ery probably act as single hammerhead structures, The minus-strand ham merhead structure of csc RNA1 is exceptional in having a central loop with only 11 conserved nucleotides, a situation previously observed in only one other natural hammerhead structure, Both hammerhead structur es of csc RNA1 are also peculiar in having an A instead of a C precedi ng the self-cleavage sites, The in vivo concentration of the plus stra nd of csc RNA1 is only slightly higher than that of its complementary strand, and significant fractions of both strands are extracted from t he tissue in the form of a complex, csc RNA1 has sequence similarities to viroids and especially to some viroid-like satellite RNAs; they al so share some characteristics of their corresponding hammerhead struct ures with these satellite RNAs, These observations, together with the association in symptomatic tissue of csc RNA1 with a set of presumably viral double-stranded RNAs, suggest that csc RNA1 is a new viroid-lik e satellite RNA.