Viroids, the Smallest Infectious Units

Viroids are infectious units that cause a number of plant diseases. They are circular molecules of RNA with molecular weights between 107,000 and 127,000. H. J. GROSS et al. sequenced the nucleotide sequence of the potato spindle tuber virus (PSTV) in 1978. It consists of 359 ribonucleotides and is characterized by numerous intramolecular base-pairings that lend stability to the structure. They are organized in a sequence of helices separated from each other by loops. The resulting structure resembles a dumb-bell with an axis ratio of 1:20. Several more viruses have been sequenced in the meantime. All of them have structures similar to that of the PSTV. They are ~240 – 380 nucleotides long and all of them have dumb-bell structures. The fact that a central portion of the molecule that is responsible for the pathogenicity of the viroids, is structurally conserved is especially interesting. PSTV-mutants with changes in this section are less pathogen than the wild type (M. SCHNÖLZER et al., 1985).

Viroids multiply even at relatively high temperature (about 35°C). Most likely, they have adapted to their host plants that have so-far strictly been found to inhabit tropical, subtropical, and continental climates. The viroids are localized within the chromatin fraction of the nucleus. The DNA-dependent RNA-polymerase II and I use the viroids as templates and produce strands that again serve as templates for the synthesis of the +strand (E. SPIESMACHER et al., 1985).