Large Satellite RNA: Molecular Parasitism or Molecular Symbiosis

Abstract

Just as relationships among cellular organisms can involve several species and various levels of dependency, so it is for subcellular agents. For viruses, the dependent member of one such relationship has been termed “satellite” based on the “satellite virus” discovered by Kassanis (1962) in some cultures of tobacco necrosis virus. Since then, satellites have been found in association with diverse viruses that infect most types of hosts (Mayo et al. 1995). The classic view of virus satellites, formed during the several years in which the only example was of satellite tobacco necrosis virus, is that they are parasitic on a virus and cause some penalty on the biological success of the helper. The satellites described in other chapters of this volume are very largely of this type. Indeed, this effect was the basis for attempts to use satellites to control diseases induced by the helper, cucumber mosaic virus. These were either by prophylactic inoculation with satellite-bearing virus (e.g., Tien Po and Chang 1983) or by transformation of plants to be protected with satellite-specific cDNA (e.g., Baulcombe et al. 1986; Harrison et al. 1987). But satellites differ in the nature of their dependency and in the degree to which the helper virus is ‘parasitized’. Some are relatively benign and seem to be well adapted to an unassuming harmonious relationship with the helper, neither helping nor hindering its multiplication.

MAM and MET are supported by funding from the Scottish Office Agriculture Environmenl and Fisheries Department