Satellite RNA depend for their multiplication on the
co-infection of a host cell by a helper virus which can itself multiply independently of the satellite. Four types of
satellite RNA have been distinguished on the basis of the size of the
RNA and what sort, if any, of
protein they encode. One of them, the B-type, comprises relatively large
RNA which are
messenger RNA for non-structural
proteins. Many of these satellites are typified by having nepoviruses as helper viruses. In general, the presence of nepovirus
mRNA satellites in a virus culture causes little or no modification to the symptoms of
infection by the helper virus and has little effect on its yield. Some satellites appear to be highly specific to a strain of helper virus but others can be helped by heterologous viruses. The
proteins encoded by nepovirus
mRNA satellites have a M(r) of 38,000 to 48,000 and are relatively basic, in particular in the N-terminal and C-terminal parts of the molecules. However, there is little similarity in amino acid sequence between
proteins encoded by different satellites and no
peptide motif could be found in all satellite
proteins. The results of reverse genetics experiments with satellites suggest that the satellite-encoded
protein is essential for the multiplication of the
satellite RNA. This system has considerable potential for the study of the mechanisms of replication both of satellite and helper virus
RNA.