Alterations in Virus Protein Synthesis and Capsid Production in Infection with DI Particles of Herpesvirus

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Abstract
High multiplicity, undiluted passage of equine herpesvirus type I (EHV-I) in [mouse fibroblast] L-M cells resulted in the rapid production of virus particles whose genome was genetically less complex, contained more reiterated DNA sequences and exhibited a greater buoyant density (.rho. = 1.724 g/ml) than the DNA (.rho. = 1.716 g/ml) of standard virus. These data and the finding that these particles inhibited the replication of standard virus in interference assays confirmed that these were defective interfering (DI) particles. Additional evidence for this was obtained from the pattern of cyclic fluctuation in infectious virus titer through 17 serial passages and from the pronounced variation in the particle to plaque ratio for each passage. Total particle production was markedly reduced in cells infected with virus preparations containing DI particles. Quantification of major cell-associated EHV-I capsid species by EM and analysis in Renografin density gradients indicated that this reduction occurred at the level of capsid assembly. Although total capsid production was reduced in cells infected with DI particle preparations, the synthesis of I (immature) capsids increased relative to that of L (empty) capsids; these alterations in the assembly of capsid species could be related to changes in the synthesis of capsid protein. In cells infected with EHV-I preparations rich in DI particles, the synthesis of major capsid protein 150,000 was greatly reduced, whereas core protein 46,000, a major component of I capsids, was overproduced as compared to standard virus infection. Capsids produced in cells infected with virus preparations rich in DI particles were identical in polypeptide composition to those made in standard virus infection.