Monday, August 20, 2012
It's not a virus, it's a viroid
A virus is a stretch of DNA or RNA, usually a few thousand bases long, enclosed in a protein shell. Once inside the cell, the RNA or DNA from the virus starts producing viral proteins, which are then used for replication.
Now imagine a circular strand of RNA that instead of a few thousand bases comprises a few hundred bases. It doesn't code for proteins, it doesn't come in a shell. And yet it's highly pathogenic and able to reproduce. In plants, that is.
A viroid is essentially a circular strand of RNA, typically between ~250 and ~450 bases long, and it doesn't encode for proteins. As a consequence, it depends entirely on cell proteins in order to replicate and propagate. Currently there are 30 known viroids, all belonging to two families, one that replicates in the cell nucleus, and one in plastids (organelles outside the nucleus) instead. While viral infection is prompted by the proteins the virus codes, it still remains a mystery how non-coding viroids can initiate phenotypic changes in their hosts. These changes are broad in degree and extent, with some viroids inducing no changes at all, and others resulting in severe pathogenicity.
"Potential pathways connecting the first viroid-host interaction that through one or more cross-talking signaling cascades ultimately lead to the macroscopic symptoms. Most components of these pathways, including the initial triggering viroid RNA species, are hypothetical ."
In , Navarro et al. explore various hypothesis as to how viroids initiate infection. One possibility is that viroid-derived RNAs could be targeting host RNA for silencing. They studied one viroid in particular that causes a severe form of albinism in the leaves, stems and fruits of peach seedlings. They deep sequenced healthy and diseased leaves and provided "direct evidence involving RNA silencing in modulation of host gene expression by a viroid."
It turns out, there's a particular human-infecting virus that behaves more like a viroid than a virus, but that story I'll save for next time. :-)
 Beatriz Navarroa,, Andreas Giselb,, Maria-Elena Rodioa,, Sonia Delgadoc,, Ricardo Floresc,, & Francesco Di Serio (2012). Viroids: How to infect a host and cause disease without encoding proteins Biochem DOI: 10.1016/j.biochi.2012.02.020