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.
Grundhoff
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A
Howard Hughes Medical Institute, Department of Microbiology, University of California Medical Center, San Francisco, California 94143-0414, USA.
Ganem
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D
D001483:Base Sequence; D002460:Cell Line; D016213:Cyclins / Q000235:genetics / Q000378:metabolism; D015967:Gene Expression Regulation, Viral; D017930:Genes, Reporter; D019288:Herpesvirus 8, Human / Q000235:genetics* / Q000502:physiology*; D008156:Luciferases / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D016366:Open Reading Frames; D010957:Plasmids / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D012270:Ribosomes / Q000378:metabolism; D014158:Transcription, Genetic; D014162:Transfection; D014764:Viral Proteins / Q000235:genetics / Q000378:metabolism; D017735:Virus Latency
D016213:Cyclins; D012333:RNA, Messenger; D014764:Viral Proteins; D008156:Luciferases