Transcriptional regulation of the ORF61 and ORF60 genes of Kaposi's sarcoma-associated herpesvirus.

The ORF61 and ORF60 genes of Kaposi's sarcoma-associated herpesvirus (KSHV) encode the ribonucleotide reductase large and small subunits, respectively. Here we show that ORF50 protein, a latent-lytic switch transactivator, activates the transcription of these two early-lytic genes through different mechanisms. Activation of the ORF61 promoter by ORF50 protein is dependent on an intact RBP-Jkappa-binding site within the identified responsive element and the expression of RBP-Jkappa protein in cells. The critical element in the ORF60 promoter in response to ORF50 was mapped to a 40-bp region. Binding of YY1, Sp1/Sp3 or unknown proteins to this element may contribute to repression or activation of the ORF60 promoter. Although ORF50 protein does not directly bind to the ORF61 and ORF60 promoters in vitro, we show the association of ORF50 protein with these two promoters in vivo. Our results provide further insights into the regulatory network of the viral lytic genes in KSHV reactivation.