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Journal of Virology, February 2001, p. 1857-1863, Vol. 75, No. 4
0022-538X/01/$04.00+0   DOI: 10.1128/JVI.75.4.1857-1863.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Mechanisms Governing Expression of the v-FLIP Gene of Kaposi's Sarcoma-Associated Herpesvirus

Adam Grundhoff and Don Ganem*

Howard Hughes Medical Institute, Departments of Microbiology and Medicine, University of California Medical Center, San Francisco, California 94143-0414

Received 12 September 2000/Accepted 20 November 2000

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.

* Corresponding author. Mailing address: Department of Microbiology and Immunology, University of California Medical Center, 513 Parnassus Ave., HSE 405, San Francisco, CA 94143-0414. Phone: (415) 476-2826. Fax: (415) 476-0939. E-mail: ganem{at}cgl.ucsf.edu.

Journal of Virology, February 2001, p. 1857-1863, Vol. 75, No. 4
0022-538X/01/$04.00+0   DOI: 10.1128/JVI.75.4.1857-1863.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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