The role of the L4 33K gene in adenovirus infection.
The late phase of adenovirus infection is characterized not only by the synthesis of late proteins and the assembly of new virions, but also by the inhibition of early gene expression and host cell translation. Previous work has demonstrated that both of these inhibitory effects depend upon expression from the major late transcription unit (MLTU), controlled by the major late promoter (MLP). Furthermore, the repression of early gene expression has been shown to be mediated in trans, suggesting a role for one or more MLTU-encoded soluble factor(s). A possible candidate for such a factor is the L4-encoded 33K gene product, a protein conserved throughout the Mastadenoviridae, but of no known function. To test the role of this protein in viral infection, a stop codon was placed at the 20th position of the 33K ORF. Viable virus with genomes containing the mutation were recovered in an overlap recombination assay. Phenotypic analysis revealed that the mutant virus had a significant deficiency in both kinetics of replication and final yield, as compared to the wild-type virus. Detailed analysis of infected cells showed that there was no detectable change in the regulation of expression of several early genes and the pIX gene. This suggests either that 33K is not involved in this late phase phenomenon or that this function is replaceable by another late protein(s). Late protein synthesis and accumulation were similar to those in wild-type-infected cells. However, the reduced yield of infectious mutant virus could be accounted for by a marked deficiency in the accumulation of intermediate particles and completed capsids, suggesting a role for 33K in the process of assembly. In addition there was a small but reproducible deficiency in the shutoff of host cell translation. These results show that the 33K protein plays an important, although apparently not essential, function in the late phase of virus infection.