Isolation and characterization of mutants of vaccinia virus with a modified 94-kDa inclusion protein

We have characterized one of the most highly expressed genes of vaccinia virus, WR strain, in the wild type and in several spontaneous mutants isolated from persistently infected cells. This gene encodes the 94-kDa inclusion protein, which is the vaccinia virus counterpart of the 160-kDa A-type inclusion (ATI) protein of cowpox virus. The homology index between both genes is greater than 95%. A deletion of two consecutive adenylate residues is responsible for a frameshift mutation and premature translational termination in the vaccinia virus gene. In addition, several point mutations and small deletions occur in the 94K gene. The deduced protein contains 725 amino acids, and 4 of the 10 repeated motifs present in the carboxyl terminus of the cowpox virus 160-kDa protein are conserved. In several mutants independently isolated from untreated and interferon-treated persistently infected cells, the gene encodes a 40-kDa protein. In mutant 87-4, this truncated protein is due to the insertion of a cytidilate residue that produces a frameshift mutation and premature translational termination. The deduced protein contains 366 amino acids and has lost all the repetitions. Transcriptional analysis has shown that the steady-state levels of mRNAs in cells infected with the mutants or wild-type vaccinia virus are similar. However, the accumulation of this protein in cells infected with the mutants is reduced indicating some instability. In addition the mutated protein is not recognized by polyclonal antisera. Existence of tandemly repeated sequences at the carboxyl terminus of this family of inclusion proteins correlates with their antigenicity. These results indicate a high degree of mutability of the ATI gene and products, which apparently has no consequence on replication in vitro, but could have relevance to control of the infection by immune responses in animal hosts.