In 1971, Theodor Diener proposed the name viroid to describe a newly discovered pathogen of potatoes. Viroids are many times smaller than the smallest virus, and consist solely of a small circle of ssRNA containing

A viroid is a plant pathogen that comprises only ssRNA and does not code for a protein product.

some 300-400 nucleotide bases and no protein coat. Enzymes in the host's nucleus are used to replicate the RNA, which does not appear to be translated into protein. Appreciable sequence homology suggests that viroids arose from transposable elements (see Chapter 11), segments of DNA capable of movement within or between DNA molecules. To date, viroids have only been found in plants, where they cause a variety of diseases.

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