Efficient genetic modification of herpesviruses such as Kaposi's sarcoma-associated herpesvirus (KSHV) has come to rely on bacterial artificial chromosome (BAC) technology. In order to facilitate this approach, we generated a new KSHV BAC clone, called BAC16, derived from the rKSHV.219 virus, which stems from KSHV and Epstein-Barr virus-coinfected JSC1 primary effusion lymphoma (PEL) cells. Restriction enzyme and complete sequencing data demonstrate that the KSHV of JSC1 PEL cells showed a minimal level of sequence variation across the entire viral genome compared to the complete genomic sequence of other KSHV strains. BAC16 not only stably propagated in bothEscherichia coliand mammalian cells without apparent genetic rearrangements, but also was capable of robustly producing infectious virions (∼5 × 107/ml). We also demonstrated the utility of BAC16 by generating deletion mutants of either the K3 or K5 genes, whose products are E3 ligases of the membrane-associated RING-CH (MARCH) family. While previous studies have shown that individual expression of either K3 or K5 results in efficient downregulation of the surface expression of major histocompatibility complex class I (MHC-I) molecules, we found that K5, but not K3, was the primary factor critical for the downregulation of MHC-I surface expression during KSHV lytic reactivation or followingde novoinfection. The data presented here demonstrate the utility of BAC16 for the generation and characterization of KSHV knockout and mutant recombinants and further emphasize the importance of functional analysis of viral genes in the context of the KSHV genome besides the study of individual gene expression.

Summary Innate immune recognition of foreign nucleic acids induces protective interferon responses. Detection of cytosolic DNA triggers downstream immune signaling through activation of cyclic GMP–AMP synthase (cGAS). We report here the crystal structure of human cGAS, revealing an unanticipated zinc-ribbon DNA-binding domain appended to a core enzymatic nucleotidyl transferase scaffold. The catalytic core of cGAS is structurally homologous to the RNA sensing enzyme, 2′–5′ oligo-adenylate synthase (OAS), and divergent C-terminal domains account for specific ligand-activation requirements of each enzyme. We show that the cGAS zinc-ribbon is essential for STING-dependent induction of the interferon response, and conserved amino acids displayed within the intervening loops are required for efficient cytosolic DNA recognition. These results demonstrate that cGAS and OAS define a new family of innate immunity sensors, and that structural divergence from a core nucleotidyl transferase enables second-messenger responses to distinct foreign nucleic acids.