Mechanisms governing expression of the v-FLIP gene of Kaposi's sarcoma-associated herpesvirus

Open reading frame 71 (ORF 71) of Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a death effector domain-containing protein that is homologous to cellular FLIPs (FLICE-inhibitory proteins) and is proposed to inhibit Fas-mediated apoptosis, Transcripts bearing ORF 71 (V-FLIP) sequences are present in all latently infected cells, However, mapping studies reveal these to be bi- or tricistronic mRNAs with ORF 71 located 3' to ORFs 72 (v-cyclin) and 73 (latency-associated nuclear antigen), raising the question of how efficient expression of V-FLIP is achieved. We explored this question by examining the expression of model bicistronic (v-cyclin/LUC) transcripts in which a luciferase (LUC) reporter replaced v-FLIP coding sequences, SLK spindle cells transfected with such constructs efficiently expressed luciferase from the 3' position, and this expression was independent of the expression of the 5' v-cyclin gene. Surprisingly, transcript mapping showed that in these cultures, efficient splicing occurred to remove v-cyclin sequences and generate monocistronic LUC transcripts. Similar splicing events produced monocistronic v-FLIP transcripts in KSHV-infected primary effusion lymphoma cells. However, these RNAs were of low abundance and were inducible by treatment with 12-O-tetradecanoylphorbol-13-acetate. Examination of the more abundant bicistronic latent RNAs revealed the presence of an efficient internal ribosome entry site (IRES) overlapping ORF 72 coding sequences. Thus, two potential mechanisms exist for v-FLIP expression, but the evidence suggests that IRES-mediated internal translational initiation on latent polycistronic mRNAs is the principal source of v-FLIP in latency.