Interference with replication of two double-stranded RNA viruses by production of N-terminal fragments of capsid polypeptides

It is possible to interfere with the replication of a number of plant RNA viruses by systemic production of viral capsid polypeptides or RNA-dependent RNA polymerases, or by production of untranslatable portions of viral plus strands or minus strands. Interference can occur by a number of mechanisms. We have discovered that the Saccharomyces cerevisiae double-stranded RNA viruses ScVL1 and ScVLa, which exist as permanent persistent infections of their host cells, can be cured very efficiently by production of N-terminal fragments of their capsid polypeptides. These totiviruses produce only two polypeptides: a capsid polypeptide (Cap) and a Cap-Pol fusion polypeptide with RNA-dependent RNA polymerase activity. Three types of interference can be detected: interference due to overproduction of both Cap and Cap-Pol, interference due to overproduction of Cap (and consequent distortion of the Cap to Cap-Pol ratio), and interference due to negative complementation by N-terminal fragments of Cap. Some N-terminal fragments of Cap appear to be incorporated into viral particles, but only in the presence of a complete Cap protein. We postulate that incorporation of N-terminal fragments of Cap results in the formation of defective particles.