Studies on sequence homology between the RNA's of alfalfa mosaic virus

Abstract

Four major and at least ten minor RNA species are present in preparations of alfalfa mosaic virus (AMV). To study the base sequence homology of these RNA's by competition hybridization, virus-specific double-stranded RNA (ds-RNA) was isolated from infected leaves. Determination of the fractions of 3H-labeled AMV-RNA displaced from the hybrid by purified preparations of the unlabeled major RNA's (B-, M-, Tb- and Ta-RNA) revealed the presence of unique base sequences in the genome fragments B-, M- and Tb-RNA. The size of the base sequences common to B- and M-RNA, if any, is less than 200 nucleotides. A possible redundancy between Tb-RNA and B- or M-RNA could not be established because of a contamination of the Tb-RNA preparation with minor RNA species. A preparation of unlabeled minor RNA species was able to displace all 3H-labeled AMV-RNA from the hybrid, showing the presence of all virus-specific base sequences in this material. Unlabeled preparations of Ta- and Tb-RNA completely interfered with the annealing of 3H-labeled Ta-RNA to ds-RNA. This confirms the conclusion of earlier work that the Ta-RNA sequences are present in Tb-RNA. However, evidence was obtained indicating that Ta-RNA is heterogeneous and contains all Ta-RNA sequences. The nature of the small interference of B- and M-RNA with the annealing of the 3H-labeled Ta-RNA preparation to ds-RNA is discussed. A comparison between the base sequences of four AMV strains revealed an almost complete homology. This may be correlated with the fact that components of different strains can be exchanged without loss of infectivity. No base sequence homology was observed between AMV and other viruses with a tripartite genome: brome mosaic virus, cowpea chlorotic mottle virus, cucumber mosaic virus, and tobacco streak virus.