Structure
Viroids are infectious agents
composed exclusively of a single piece of circular single stranded RNA
which has some double-stranded regions. 
Because of their simplified structures both prions and viroids are sometimes called subviral particles. Viroids mainly cause plant diseases but have recently been reported to cause a human disease.
Catalytic RNAs are those
that have the intrinsic ability to break and form covalent bonds; Viroids
are catalytic RNA's (ribozymes) that cleave RNA to produce fragments
containing a 5'-hydroxyl and a 2', 3'-cyclic phosphate.
This is a nonhydrolytic reaction in which the same number of phosphodiester bonds are maintained and the transesterification reaction is theoretically reversible. This reaction is considered to play an essential role in the replication of these RNAs in vivo. Such reactions are all intramolecular and hence quasi-catalytic with single turnover. These RNAs can be manipulated, however, to provide true catalytic cleavage in trans-reactions.
Replication
Circular, pathogenic RNAs are replicated by a rolling circle mechanism in vivo. There are two variations of this rolling circle mechanism:
In the first variation (A), the circular plus strand is copied by viroid RNA-dependent RNA polymerase to form a concatameric minus strand (step 2). Site-specific cleavage (arrows) of this strand produces a monomer that is circularized by a host RNA ligase (step 3) and then copied by the RNA polymerase to produce a concatameric plus strand. Cleavage of this strand (step 5) produces monomers which, on circularization, produces the progeny circular, plus RNA, the dominant form in vivo.
In the other variation (B), the concatameric minus strand of step 1 is not cleaved but is copied directly to give a concatameric plus strand (step 3), which is cleared specifically to monomers for ligation to the circular progeny. Those RNAs that self-cleave only in the plus strand in vitro are considered to follow this route.
The hepatitis D viroid genome is a minus strand that gives rise to two RNA species. One of these is a mRNA for the delta antigen and the other is a complete complimentary copy (plus strand or anti-genome). The anti-genome acts as a template to make more minus strands. The minus strand self-cleaves and self-ligates. HDV replication takes place in the nucleus but delta antigen is made in the cytoplasm. The delta antigen is the only protein made by the HDV mRNA. It has a +12 charge at physiologic pH, accumulates in the nucleus and binds to minus strand RNA as a dimer. The delta antigen is necessary for viroid assembly but its exact mode of action is unknown.
Human pathologies induced by viroids
The only human disease known to be caused by a viroid is hepatitis D. This disease was previously ascribed to a defective virus called the delta agent. However, it now is known that the delta agent is a viroid enclosed in a hepatitis B virus capsid. For hepatitis D to occur there must be simultaneous infection of a cell with both the hepatitis B virus and the hepatitis D viroid. There is extensive sequence complementarity between the hepatitis D viroid RNA and human liver cell 7S RNA, a small cytoplasmic RNA that is a component of the signal recognition particle, the structure involved in the translocation of secretory and membrane-associated particles. The hepatitis D viroid causes liver cell death via sequestering this 7S RNA and/or cleaving it.Transmission
The hepatitis D viroid can only enter a human liver cell if it is enclosed in a capsid that contains a binding protein. It obtains this from the hepatitis B virus. The delta agent then enters the blood stream and can be transmitted via blood or serum transfusions.
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