Multiplication/Replication of Viroids

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 Multiplication/Replication

Viroids multiply inside the nucleus (Pospiviroidae) or chloroplasts (Avsunviroidae) of infected plant cell by ‘Rolling Circle’ mechanism, using the RNA polymerase enzyme of host. cell. Viroids require RNA polymerase II, a host cell enzyme normally associated with synthesis of messenger RNA from DNA, which instead catalyzes "rolling circle" synthesis of new RNA using the viroid as template. The resulting long RNA molecule is produced which cut into pieces (monomers) and then ligated either the monomers (autocatalytically or by cellular factors depending on a viroid) to form an individual circular viroid.

There are two variations of this rolling circle mechanism: 

(A) In the first variation,
i) the circular plus (+) strand is copied by viroid RNA-dependent RNA polymerase to form a concatemer minus (-) strand (step 2).
ii) 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.
iii) Cleavage of this strand (step 5) produces monomers which, on circularization, produces the progeny circular, plus RNA, the dominant form in vivo.


(B) In the other variation,
i) 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.
After replication, the progeny RNA moves to neighbor healthy cells through the connecting plasmodesmata or to distant parts of the plant via the phloem. They adopt the host RNA Polymerase II which synthesizes a negative RNA strand. Then it serves as a template for the synthesis of the other viriod RNAs. So in a sense, at least some viroids are ribozymes.

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