Cis-acting functions of alfalfa mosaic virus proteins involved in replication and encapsidation of viral RNA.

cDNA clones of RNAs 1 and 2 of alfalfa mosaic virus (AMV) were slightly modified to permit transcription of infectious RNAs with T7 RNA polymerase. Together with transcripts of an available clone of AMV RNA 3, these transcripts were used to study cis- and trans-acting functions of AMV proteins in protoplasts from nontransgenic tobacco plants and from plants transformed with the P1 and P2 genes, encoded by RNAs 1 and 2, respectively. Transgenic P1 was unable to complement mutations in the P1 gene in RNA 1, pointing to a cis-acting function of P1 in RNA 1 replication. A study of the replication of RNA 3 mutants in nontransgenic protoplasts revealed that coat protein (CP) expressed from RNA 3 in the inoculum is required in trans for replication and encapsidation of RNAs 1 and 2 but is required in cis for replication and encapsidation of RNA 3. CP is required in the inoculum to initiate infection of nontransgenic plants and protoplasts. When protoplasts expressing both P1 and P2 (P12 protoplasts) were infected with RNAs 1, 2, and 3, initiation of replication of RNAs 1 and 2 required the presence of CP in the inoculum, whereas the initiation of replication of RNA 3 did not. This demonstrated that CP expressed from RNA 3 cannot substitute for the early function of CP in the inoculum. The results showed that CP in the inoculum is required to permit viral minus-strand RNA synthesis, whereas CP expressed from RNA 3 after the initiation of infection is required for plus-strand RNA synthesis. Copyright 1999 Academic Press.