Preferential replication of defective turnip yellow mosaic virus RNAs that express the 150-kDa protein in cis.

The turnip yellow mosaic virus genome encodes two proteins (the 150-kDa and 70-kDa proteins) that are proteolytically released from a single precursor and which are essential for RNA replication. Genomes with mutations in either of these coding regions were defective for independent replication in turnip protoplasts. The replication in trans of genomes with mutations in each region was studied by coinoculation with either a helper genome that carries a deletion in the coat protein gene, or with a second defective RNA that carries a mutation in the region encoding the other essential protein. Inefficient trans-replication of the defective RNAs was observed in most cases. In contrast, a defective RNA with a large deletion in the 70-kDa protein coding region could be replicated efficiently in trans, demonstrating that the cis-preference of replication can be overcome in some cases. Defective RNAs encoding wild type 150-kDa protein and defective 70-kDa protein were more efficiently replicated in trans than those encoding defective 150-kDa protein and wild type 70-kDa protein. The results suggest a model in which the 150-kDa and 70-kDa proteins form a relatively stable complex in cis on the viral RNA template.