Cis- and trans-acting elements in cowpea mosaic virus RNA replication.

Cowpea mosaic virus (CPMV) B-RNA encodes the viral proteins required for viral RNA replication while M-RNA does so for the capsid proteins and functions required in cell-to-cell movement of the virus. Accordingly, B-RNA can replicate by itself, whereas M-RNA can only replicate in the presence of B-RNA. We have made heterologous sequence insertions at different positions in the open reading frame of B-RNA, leaving the 5' and 3' non-coding ends intact. None of these mutant B-RNAs were able to replicate. Furthermore, it was not possible to support replication of these mutant B-RNAs by co-inoculating wild-type B-RNA as a helper, indicating that B-RNA can not be replicated in trans. In contrast, replication of M-RNA must occur in trans, as the viral replicative proteins are encoded by B-RNA. Mutant M-RNA transcripts containing 5' and 3' non-coding regions of B-RNA are still efficiently replicated in protoplasts if co-inoculated with B-RNA, indicating that in cis or in trans replication of the CPMV RNAs is not primarily determined by the non-coding regions. Remarkably, for replication of M-RNA, the N-terminal domain of the 58K protein encoded by M-RNA was found to be required.