Targeted disruption of Kaposi's sarcoma-associated herpesvirus ORF57 in the viral genome is detrimental for the expression of ORF59, K8alpha, and K8.1 and the production of infectious virus.

Kaposi's sarcoma-associated herpesvirus (KSHV) ORF57 regulates viral gene expression at the posttranscriptional level during viral lytic infection. To study its function in the context of the viral genome, we disrupted KSHV ORF57 in the KSHV genome by transposon-based mutagenesis. The insertion of the transposon into the ORF57 exon 2 region also interrupted the 3' untranslated region of KSHV ORF56, which overlaps with the ORF57 coding region. The disrupted viral genome, Bac36-Delta57, did not express ORF57, ORF59, K8alpha, K8.1, or a higher level of polyadenylated nuclear RNA after butyrate induction and could not be induced to produce infectious viruses in the presence of valproic acid, a histone deacetylase inhibitor and a novel KSHV lytic cycle inducer. The ectopic expression of ORF57 partially complemented the replication deficiency of the disrupted KSHV genome and the expression of the lytic gene ORF59. The induced production of infectious virus particles from the disrupted KSHV genome was also substantially restored by the simultaneous expression of both ORF57 and ORF56; complementation by ORF57 alone only partially restored the production of virus, and expression of ORF56 alone showed no effect. Altogether, our data indicate that in the context of the viral genome, KSHV ORF57 is essential for ORF59, K8alpha, and K8.1 expression and infectious virus production.