Expression of the Middle Component RNA of Cowpea Mosaic Virus in vivo By JOAN WELL INK, 1. MARTINE JAEGLE, l t HE INRICH PR INZ, 2

Upon infection of cowpea mesophyll protoplasts with cowpea mosaic virus (CPMV), the only M RNA-encoded proteins detected so far have been the two capsid proteins VP37 and VP23. We now report he detection of a Mr 60000 (60K) precursor to both capsid proteins in infected protoplasts cultured in the presence of zinc ions. Furthermore a M RNA-encoded 48K protein was detected in the membrane fraction of infected cells using antisera raised against synthetic peptides. The results obtained indicate that, just like the bottom component RNA, the M RNA of CPMV is translated in vivo into a polyprotein from which proteins are derived by proteolytic cleavage