REPLICATION AND TRANSLATION OF COWPEA MOSAIC-VIRUS RNAS ARE TIGHTLY LINKED

The genome of cowpea mosaic virus (CPMV) is divided among two positive strand RNA molecules. B-RNA is able to replicate independently from M -RNA in cowpea protoplasts. Replication of mutant B-transcripts could not be supported by co-inoculated wild-type B-RNA, indicating that B-R NA cannot be efficiently replicated in trans. Hence replication of a B -RNA molecule is tightly linked to its translation and/or at least one of the replicative proteins functions in cis only. Remarkably also fo r efficient replication of M-RNA one of its translation products was f ound to be required in cis. This 58K protein possibly helps in directi ng the B-RNA-encoded replication complex to the M-RNA. In order to ide ntify the viral polymerase the CPMV B-RNA-specific proteins have been produced individually in cowpea protoplasts using CaMV 35S promoter ba sed expression vectors. Only protoplasts transfected with a vector con taining the 200K coding sequence were able to support replication of c o-transfected M-RNA. Despite this, CPMV-specific RNA polymerase activi ty could not be detected in extracts of these protoplasts using a poly (A)/oligo(U) assay. These results indicate that, in contrast to the po liovirus polymerase, the CPMV polymerase is not able to accept oligo(U ) as a primer and in addition support the concept that translation and replication are linked.