Viroids, infectious agents that encode no proteins

Genomes of non-defective viruses range in size from 2,400,000 bp of dsDNA (Pandoravirus salinus) to 1,759 bp of ssDNA (porcine circovirus). Are even smaller viral genomes possible? The subviral agents called viroids provide an answer to this question. Viroids, the smallest known pathogens, are naked, circular, single-stranded RNA molecules that do not encode protein yet replicate autonomously when introduced into host plants. Potato spindle tuber viroid, discovered in 1971, is the prototype; 29 other viroids have since been discovered ranging in length from 120 to 475 nucleotides. Viroids only infect plants; some cause economically important diseases of crop plants, while others appear to be benign. Two examples of economically important viroids are coconut cadang-cadang viroid (which causes a lethal infection of coconut palms) and apple scar skin viroid (which causes an infection that results in visually unappealing apples). The 30 known viroids have been classified in two families. Members of the Pospiviroidae, named for potato spindle tuber viroid, have a rod-like secondary structure with small single stranded regions, a central conserved region, and replicate in the nucleus (illustrated; click to enlarge; figure credit). The Avsunviroidae, named for avocado sunblotch viroid, have both rod-like and branched regions, but lack a central conserved region and replicate in chloroplasts. In contrast to the Pospiviroidae, the latter RNA molecules are functional ribozymes, and t...
Source: virology blog - Category: Virology Authors: Tags: Basic virology Information DNA dependent RNA polymerase noncoding RNA plant potato spindle tuber viroid ribozyme siRNA viral virus Source Type: blogs