The site of cleavage of the small coat protein of cowpea mosaic virus has been precisely mapped and the proteolysis has been shown to result in the loss of 24 amino acids from the carboxyl-terminus of the protein. A series of premature termination and deletion mutants was constructed to investigate the role or roles of these carboxyl-terminal amino acids in the viral replication cycle. Mutants containing premature termination codons at or downstream of the cleavage site were viable but reverted to wild-type after a single passage through cowpea plants, indicating that the carboxyl-terminal amino acids are important. Mutants with the equivalent deletions were genetically stable and shown to be debilitated with respect to virus accumulation. The specific infectivity of preparations of a deletion mutant (DM4) lacking all 24 amino acids was 6-fold less than that of a wild-type preparation. This was shown to be a result of DM4 preparations containing a much increased percentage (73%) of empty (RNA-free) particles, a finding that implicates the cleavable carboxyl-terminal residues in the packaging of the virion RNAs.
Copyright 1999 Academic Press.