Viroid RNA redirects host DNA ligase 1 to act as an RNA ligase.

Viroids are a unique class of noncoding RNAs: composed of only a circular, single-stranded molecule of 246-401 nt, they manage to replicate, move, circumvent host defenses, and frequently induce disease in higher plants. Viroids replicate through an RNA-to-RNA rolling-circle mechanism consisting of transcription of oligomeric viroid RNA intermediates, cleavage to unit-length strands, and circularization. Though the host RNA polymerase II (redirected to accept RNA templates) mediates RNA synthesis and a type-III RNase presumably cleavage of Potato spindle tuber viroid (PSTVd) and closely related members of the family Pospiviroidae, the host enzyme catalyzing the final circularization step, has remained elusive.

Involvement of the chloroplastic isoform of tRNA ligase in the replication of viroids belonging to the family Avsunviroidae.

Avocado sunblotch viroid, peach latent mosaic viroid, chrysanthemum chlorotic mottle viroid, and eggplant latent viroid (ELVd), the four recognized members of the family Avsunviroidae, replicate through the symmetric pathway of an RNA-to-RNA rolling-circle mechanism in chloroplasts of infected cells. Viroid oligomeric transcripts of both polarities contain embedded hammerhead ribozymes that, during replication, mediate their self-cleavage to monomeric-length RNAs with 5'-hydroxyl and 2',3'-phosphodiester termini that are subsequently circularized. We report that a recombinant version of the chloroplastic isoform of the tRNA ligase from eggplant (Solanum melongena L.

A chloroplastic RNA ligase activity analogous to the bacterial and archaeal 2´-5' RNA ligase.

Bacteria and archaea contain a 2'-5' RNA ligase that seals in vitro 2',3'-cyclic phosphodiester and 5'-hydroxyl RNA termini, generating a 2',5'-phosphodiester bond. In our search for an RNA ligase able to circularize the monomeric linear replication intermediates of viroids belonging to the family Avsunviroidae, which replicate in the chloroplast, we have identified in spinach (Spinacea oleracea L.) chloroplasts a new RNA ligase activity whose properties resemble those of the bacterial and archaeal 2'-5' RNA ligase.

Rolling-circle replication of viroids, viroid-like satellite RNAs and hepatitis delta virus: variations on a theme.

Viroids and viroid-like satellite RNAs from plants, and the human hepatitis delta virus (HDV) RNA share some properties that include small size, circularity and replication through a rolling-circle mechanism. Replication occurs in different cell compartments (nucleus, chloroplast and membrane-associated cytoplasmatic vesicles) and has three steps: RNA polymerization, cleavage and ligation. The first step generates oligomeric RNAs that result from the reiterative transcription of the circular templates of one or both polarities, and is catalyzed by either the RNA-dependent RNA polymerase of the helper virus on which viroid-like satellite RNAs are functionally dependent, or by host DNA-dependent RNA polymerases that, remarkably, viroids and HDV redirect to transcribe RNA templates.

Ribosomal protein L5 and transcription factor IIIA from Arabidopsis thaliana bind in vitro specifically Potato spindle tuber viroid RNA.

Potato spindle tuber viroid (PSTVd) contains an element of tertiary structure -loop E- also present in eukaryotic 5S rRNA. The ribosomal protein L5 and transcription factor IIIA (TFIIIA) from Arabidopsis thaliana bind 5S rRNA in vitro and in vivo, mediating different functions that include nucleocytoplasmic transport and transcription activation, respectively. We show that A.

Viroid replication: rolling-circles, enzymes and ribozymes.

Viroids, due to their small size and lack of protein-coding capacity, must rely essentially on their hosts for replication. Intriguingly, viroids have evolved the ability to replicate in two cellular organella, the nucleus (family Pospiviroidae) and the chloroplast (family Avsunviroidae). Viroid replication proceeds through an RNA-based rolling-circle mechanism with three steps that, with some variations, operate in both polarity strands: i) synthesis of longer-than-unit strands catalyzed by either the nuclear RNA polymerase II or a nuclear-encoded chloroplastic RNA polymerase, in both instances redirected to transcribe RNA templates, ii) cleavage to unit-length, which in the family Avsunviroidae is mediated by hammerhead ribozymes embedded in both polarity strands, while in the family Pospiviroidae the oligomeric RNAs provide the proper conformation but not the catalytic activity, and iii) circularization.