Three viral proteins, all products of early region 2 (E2), participate directly in adenovirus DNA replication. Three products of early region 4 (E4) also affect viral DNA synthesis: the product of E4 ORF4 inhibits viral DNA accumulation, while the products of E4 ORFs 3 and 6 antagonize that effect of ORF4 expression. Because no E4 products are required for DNA synthesis, these proteins probably act indirectly. The E4 ORF3, 4, and 6 proteins all participate in aspects of the regulation of viral gene expression. To determine whether they modulate DNA replication by effects on expression of viral replication proteins, we examined E2 expression in E4 mutant-infected cells. In cells infected by ORF3-, 6- mutants, expression of ORF4 substantially depressed the steady-state levels of replication proteins and E2 mRNAs, reduced E2 transcription rates, and profoundly inhibited viral DNA replication. Thus, in the absence of E4 ORFs 3 and 6, ORF4 acts as a transcriptional regulator of E2 expression, and reduced replication protein levels largely account for the inhibition of DNA replication by ORF4. Cells infected by viruses that express ORFs 3 and 6 in addition to ORF4 accumulated much larger quantities of viral DNA than did cells infected by the ORF3-, 6-, 4+ mutant. Increased DNA accumulation was not accompanied by a comparable increase in E2 expression. Therefore, the ORF3 and 6 products counteract the ORF4-induced reduction of DNA replication by a mechanism other than reversing the inhibitory effect of ORF4 on E2 expression. The effect of ORF4 on E2 expression is consistent with its ability to regulate levels of the transcription factor AP-1 (Muller et al., 1992, J. Virol. 66, 5867-5878); the mechanism by which ORFs 3 and 6 enhance replication is unknown.
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