Introduction

Satellite viruses and satellite RNAs are sub-viral microbes which depend on another virus, referred to as the ‘helper virus’, for their replication within a host cell. The basis of satellite dependence is not completely understood, but it is presumed that satellite genomes are replicated by the RNA-dependent RNA polymerase encoded by the helper virus. Along with viroids, satellites are the smallest and simplest biological entities known, with single-stranded RNA genomes ranging from 194 to 1376 nt. Satellites are uniquely prevalent in plant hosts, and have been found associated with 28 different plant viruses from diverse virus groups (Roossinck, Sleat & Palukaitis, 1992). To date, no examples of satellites of animal viruses are known. The only satellite-like entity associated with an animal virus, hepatitis delta virus, is not a true satellite because it is capable of autonomous replication, and appears to need its helper virus only for encapsidation and transmission (Taylor, 1990).

The classical definition of a satellite involves three main features (Murant & Mayo, 1982). These are that satellites are not capable of replicating in the absence of the helper virus, they are not required for the life cycle of the helper virus, and that they share little or no sequence similarity with the helper virus genome. Thus, satellites differ from other sub viral entities such as viroids, which are capable of independent replication in a host cell, and defective interfering (DI) RNAs, which are comprised of helper virus genetic sequences. Satellites often interfere with the replication of their helper viruses, and the symptoms caused by helper virus/satellite co-infections can differ dramatically from those of the helper virus alone.