ORF50-dependent and ORF50-independent activation of the ORF45 gene of Kaposi's sarcoma-associated herpesvirus

• The KSHV ORF50 protein is able to activate the ORF45 gene expression.
• Activation of the ORF45 promoter by ORF50 is mediated through RBP-Jκ.
• The ORF45 gene can be also induced by sodium butyrate independently of ORF50.
• The NF-Y and Sp1-binding sites are required for the butyrate-induced activation.

The ORF45 gene of Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a multifunctional tegument protein. Here, we characterize the transcriptional control of the ORF45 gene and show that its promoter can be activated by ORF50 protein, a latent-lytic switch transactivator. The ORF45 promoter can also be induced by sodium butyrate (SB), a histone deacetylase inhibitor, in the absence of ORF50 protein. Although SB induces the ORF45 gene independently of ORF50, its full activation may require the presence of ORF50. Deletion and point mutation analyses revealed that two RBP-Jκ-binding sites in the ORF45 promoter confer the ORF50 responsiveness, whereas NF-Y and Sp1-binding sites mediate the response to SB. Direct binding of NF-Y, Sp1, or RBP-Jκ protein to the ORF45 promoter is required for the promoter activation induced by SB or by ORF50. In conclusion, our study demonstrates both ORF50-dependent and ORF50-independent transcriptional mechanisms operated on the activation of the ORF45 gene.