نبذة مختصرة : The mechanism regulating expression of late genes, encoding viral structural components, is an unresolved problem in the biology of DNA tumor viruses. Here we show that BGLF4, the only protein kinase encoded by Epstein-Barr virus (EBV), controls expression of late genes independent of its effect on viral DNA replication. Ectopic expression of BGLF4 in cells lacking the kinase gene stimulated the transcript levels of six late genes by 8- to 10-fold. Introduction of a BGLF4 mutant that eliminated its kinase activity did not stimulate late gene expression. In cells infected with wild-type EBV, siRNA to BGLF4 (siG4) markedly reduced late gene expression without compromising viral DNA replication. Synthesis of late products was restored upon expression of a form of BGLF4 resistant to the siRNA. Studying the EBV transcriptome using mRNA-seq during the late phase of the lytic cycle in the absence and presence of siG4 showed that BGLF4 controlled expression of 31 late genes. Analysis of the EBV transcriptome identified BGLF3 as a gene whose expression was reduced as a result of silencing BGLF4. Knockdown of BGLF3 markedly reduced late gene expression but had no effect on viral DNA replication or expression of BGLF4. Our findings reveal the presence of a late control locus encompassing BGLF3 and BGLF4 in the EBV genome, and provide evidence for the importance of both proteins in post-replication events that are necessary for expression of late genes.
Author Summary Epstein-Barr virus (EBV) is linked to the development of several types of cancer. Synthesis of structural proteins, a group of proteins that forms the protein shell around the viral genome, is essential for EBV infection and pathogenesis. Genes encoding structural proteins are expressed late in the viral life cycle after amplification of the viral genome. The mechanism controlling expression of this group of proteins represents a longstanding conundrum in EBV and other DNA viruses. In this report, we demonstrate that two EBV regulatory proteins control synthesis of mRNAs encoding viral structural proteins. These two proteins are: BGLF4, a protein kinase conserved in all herpesviruses, and BGLF3, a protein of unknown function with no cellular counterparts. We present evidence that the enzymatic activity of BGLF4 is required after replication of viral DNA to stimulate expression of structural proteins. BGLF3 and BGLF4 are expressed from the same locus in the genome; the two proteins work in concert and independently to promote expression of viral genes encoding structural proteins. Our findings provide novel insights into control of expression of genes encoding viral structural proteins. The enzymatic activity of BGLF4 is a potential target for development of new antiviral drugs.
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