Abstract
Viroids are small infectious RNA molecules that replicate in plants via RNA-RNA replication processes. The molecular mechanism responsible for this replication has attracted great interest, and studies on this topic have yielded interesting biological findings on the processes in which RNA is involved. Viroids belonging to the Avsunviroidae family replicate in the chloroplasts of infected hosts. It has by now been established that chloroplasts and cyanobacteria share a common have ancestor. In view of this phylogenetic relationship, we investigated whether a member of the Avsunviroidae family could be replicated in a cyanobacterium. The results obtained here show that Avocado Sunblotch Viroid (ASBVd) RNA is able to replicate in the filamentous cyanobacterium Nostoc PCC 7120. Indeed, Northern blot hybridization showed that linear forms of “minus” polarity were detected in RNA extracted from Nostoc cells expressing ASBVd dimers of positive “polarity”, and that linear forms of “negative” polarity were detected in RNA extracted from Nostoc cells expressing ASBVd dimers of “positive” polarity. ASBVd replication does not impair the growth of Nostoc. These results provide the first evidence that a prokaryotic cell possesses all the machinery required to sustain the process of RNA-RNA replication. The data obtained here are of great importance, since they might shed light on the evolution of the cellular factors on which RNA replication processes depend.
Highlights
Viroids are the smallest pathogens known to exist on Earth
The results obtained here show that Avocado Sunblotch Viroid (ASBVd) RNA is able to replicate in the filamentous cyanobacterium Nostoc PCC 7120
The LK sequence alone was used with the strand-specific primer for the PCR (Figure 1 and Table 1). This procedure ensured that only DNA synthesized during the RT step could be amplified during the PCR [17], making it possible after the PCR reaction to detect the RNA-RNA replication of the ASBVd (-) strand
Summary
Viroids are the smallest pathogens known to exist on Earth They were found in the early 70s to be the causal agents responsible for the potato tubercle disease, which had been blamed so far on bacteria or viruses [1]. This pioneering study led to the discovery of a highly intriguing non coding RNA, which is able to infect a large panel of crop plants [2,3]. Viroids are composed of free RNA without any envelope or capsid They can occur in infected plant cells in two forms: in the positive polarity form, which is the most abundant, and the minus polarity one, which corresponds to a replication intermediate. The DNAdependent RNA polymerase II has been found to replicate members of the Pospiviroidae family [5], while replication of members of the Avsunviroidae family is thought to depend on the nuclear-encoded DNA-dependent RNA polymerase present in the chloroplasts [6]
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