The coat protein (CP) of tobacco streak virus (TSV) can substitute for the early function of alfalfa mosaic virus (AlMV) CP in genome activation. Replacement of the CP gene in AlMV RNA 3 with the TSV CP gene and analysis of the replication of the chimeric RNA indicated that the TSV CP could not substitute for the function of AlMV CP in asymmetric plus-strand RNA accumulation but could encapsidate the chimeric RNA and permitted a low level of cell-to-cell transport.