All characterized members of the Bromoviruses, Cucumoviruses, Ilarviruses, and also alfalfa mosaic virus (AMV) contain a tripartite genome in that the three largest of the four major encapsidated RNAs are required for infectivity. The Ilarviruses and AMV are distinguished from the two other virus groups by the additional requirement for infection of either a small amount of coat protein or its subgenomic mRNA, RNA 4 (reviewed in van Vloten-Doting and Jaspars, 1977). A further distinguishing feature is the inability of the RNAs of Ilarviruses and AMV to be aminoa-cylated in the presence of plant aminoacyl tRNA synthetases, whereas the RNAs of the Bromoviruses and Cucumoviruses that have been tested accept tyrosine (Hall, 1979; Koper-Zwarthoff and Bol, 1980). This division of the viruses into two groups on the basis of the ability or inability of their RNAs to be aminoacylated indicates significant differences in the biological and structural properties of the RNAs.