Kaposi’s sarcoma-associated herpesvirus (KSHV) is an oncogenic virus that causes Kaposi’s sarcoma and lymphoproliferative diseases primarily in immunocompromised patients. Like all herpesviruses, KSHV uses the host gene expression machinery to carefully control the timing and abundance of viral mRNAs in its latent and lytic phases. While emphasis has been placed on transcriptional control, recent reports suggest two distinct interactions between KSHV and host RNA quality control (QC) pathways that degrade nuclear transcripts. The goal of this proposal is to define the molecular mechanisms at the interface of host nuclear RNA QC pathways and KSHV gene expression. The aims will define mechanisms that KSHV uses to protect from or exploit host RNA decay pathways. In addition, some of the host pathways remain poorly understood, so the viral mechanisms will be harnessed to uncover fundamental aspects of human molecular biology. In one pathway studied here, the host RNA QC machinery targets viral transcripts for nuclear decay. To protect its RNAs, the KSHV ORF57 protein counters that pathway likely through other host factors including an RNA-binding protein called ALYREF. In Aims 1 and 2 of the current proposal, the mechanisms of ORF57-mediated protection and the RNAs targeted by specific host factors will be determined in molecular detail. Interestingly, ALYREF and other ORF57 binding proteins are host mRNA export factors. In Aim 3, their proposed roles in viral RNA stability will be tested on host RNAs to determine whether they reflect a general activity of these proteins. In addition to protection from decay, KSHV has been reported to exploit the PABPN1-PAPα/γ mediated RNA decay (PPD) pathway to control temporal expression of late genes. Aim 4 of the current proposal seeks to define the mechanisms of PPD- mediated regulation of viral gene expression and to better define the components of the PPD pathway. Taken together, the proposed KSHV studies investigate the molecular mechanisms involved in nuclear RNA QC pathways to better understand the basic gene expression mechanisms of the human pathogen KSHV and its human host cell.