Conserved Structural Motifs in the Hammerhead Ribozyme of a Chloroplast Viroid Mimic tRNA Anticodon Structure to Hijack tRNA Ligase for Viroid Circularization
Viroids belonging to the family Avsunviroidae contain hammerhead ribozymes that process to unit length the oligomeric RNAs of both polarities generated during the rolling-circle replication that occurs in chloroplasts of host plants. Linear products, with 5-hydroxyl and 2,3-phosphodiester termini, are then recognized and circularized by the host chloroplastic isoform of the tRNA ligase. Here we analyze the circularization process of eggplant latent viroid (ELVd), an asymptomatic viroid that infects eggplants (Solanum melongena L.), using an Escherichia coli co-expression system in which longer-than-unit linear ELVd (+) precursors are expressed along with the eggplant chloroplastic tRNA ligase. The RNA precursor contains two copies of the hammerhead ribozyme and yields the appropriate termini for the tRNA ligase-mediated ligation in bacteria. We have determined that the ligation efficiency is highly dependent on the presence of ribozyme sequences in the ligatable termini, since the circularization of a series of viroid variants in which the ligation position was rearranged increased substantially in the presence of these sequences. Further in silico analysis showed sequence and structure similarity between the hammerhead ribozyme catalytic pocket and the anticodon loop of tRNAs, both of which harbor a characteristic U-turn of the phosphodiester backbone. Directed mutagenesis in the ribozyme domain supports the role of this U-turn loop in the ligation process. We propose that, in addition to its self-cleavage function, the viroid ribozymes have evolved to mimic the structure of the tRNA anticodon loop to recruit host tRNA ligase for the circularization of the monomeric linear replication intermediates. IMPORTANCEViroids are a very particular class of infectious agents because they only consist of a small RNA that, to our current knowledge, does not encode for proteins. Consequently, viroids parasite host factors and structures to mediate all processes in the infectious cycle. How these small infectious RNAs are able to hijack host resources is currently a mystery. In this work, we shed some light on the functionality of hammerhead ribozymes during replication of viroids that belong to the family Avsunviroidae, which replicate in the chloroplasts. Our findings suggest that, in addition to mediate self-cleavage of replication intermediates, hammerhead ribozymes also recruit tRNA ligase for monomer circularization, likely mimicking a common host tRNA structural motif.