Altered balance of the synthesis of plus- and minus-strand RNAs induced by RNAs 1 and 2 of alfalfa mosaic virus in the absence of RNA 3.

The synthesis of viral plus-strand and minus-strand RNAs in cowpea protoplasts inoculated with mixtures of alfalfa mosaic virus nucleoproteins (B, M, Tb, and Ta) was analyzed by the Northern blotting technique. A mixture of B, M, and Tb induced the synthesis of plus-strand RNAs 1, 2, 3, and 4 and three minus-strand RNAs corresponding to RNAs 1, 2, and 3, respectively. Compared to this complete infection, a mixture of B and M induced the synthesis of a reduced amount of plus-strand RNAs 1 and 2 and a greatly enhanced amount of minus-strand RNAs 1 and 2. No detectable viral RNA synthesis was induced by mixtures of B and Tb or M and Tb. It is concluded that expression of genomic RNAs 1 and 2 results in the formation of a replicase activity that produces roughly equal amounts of viral plus- and minus-strand RNAs and that an RNA 3-encoded product, possibly the coat protein, is responsible for a switch to an asymmetric production of viral plus-strand RNA. The observation that no minus-strand corresponding to the subgenomic RNA 4 is produced suggests that recognition of the genome segments by the viral replicase involves sequences outside the 3'-terminal regions that are homologous to RNA 4.