Function of top component a RNA in the initiation of infection by alfalfa mosaic virus

Abstract

The function of top component a (Ta)-RNA in the initiation of infection by alfalfa mosaic virus (AMV) was investigated by infecting tobacco with combinations of Ta-RNA from one AMV-strain and bottom component (B)-RNA, middle component (M)-RNA, and top component b (Tb)-RNA from another strain. In these mixtures the genetic information for the viral coat protein is present in the Tb-RNA and probably also in the Ta-RNA, the latter being presumably a monocistronic messenger. No detectable amount of coat protein derived by translation of the inoculated Ta-RNA was found in the progenies of six different combinations. Subculturing of single lesions yielded isolates all having properties identical to the strain from which their B-, M- and Tb-RNA was derived. This indicates that the inoculated Ta-RNA is translated into only a few coat protein molecules, and that all Ta present in the progeny is synthesized under the direction of B, M, and Tb RNAs. The function of the coat protein in the initiation of infection is still obscure, but the fact that no replication of RNA was detected upon infection with a mixture of B-, M-, and Tb-RNA alone, suggests that in the absence of coat protein most of the viral functions are blocked.