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Journal of Virology, April 2000, p. 3586-3597, Vol. 74, No. 8
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Kaposi's Sarcoma-Associated Herpesvirus Open Reading Frame 57 Encodes a Posttranscriptional Regulator with Multiple Distinct Activitiesdagger

Jessica R. Kirshner, David M. Lukac, Jean Chang, and Don Ganem*

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

Received 4 October 1999/Accepted 21 January 2000

Open reading frame (ORF) 57 of Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a homolog of known posttranscriptional regulators that are essential for replication in other herpesviruses. Here, we examined the expression of this gene and the function(s) of its product. KSHV ORF 57 is expressed very early in infection from a 1.6-kb spliced RNA bearing several in-frame initiation codons. Its product is a nuclear protein that, in transient assays, has little effect on the expression of luciferase reporter genes driven by a variety of KSHV and heterologous promoters. However, ORF 57 protein enhances the accumulation of several viral transcripts, in a manner suggesting posttranscriptional regulation. These transcripts include not only known cytoplasmic mRNAs (e.g., ORF 59) but also a nuclear RNA (nut-1) that lacks coding potential. Finally, ORF 57 protein can also modulate the effects of the ORF 50 gene product, a classical transactivator known to be required for lytic induction. The expression from some (e.g., nut-1) but not all (e.g., tk) ORF 50-responsive promoters can be synergistically enhanced by coexpression of ORF 50 and ORF 57. This effect is not due to upregulation of ORF 50 expression but rather to a posttranslational enhancement of the transcriptional activity of ORF 50. These data indicate that ORF 57 is a powerful pleiotropic effector that can act on several posttranscriptional levels to modulate the expression of viral genes in infected cells.

* Corresponding author. Mailing address: Departments of Microbiology and Medicine, Howard Hughes Medical Institute, University of California, San Francisco, CA 94143. Phone: (415) 476-2826. Fax: (415) 476-0939. E-mail: ganem{at}cgl.ucsf.edu.

dagger This paper is dedicated to the memory of Rob Sadler, our colleague and friend.

Journal of Virology, April 2000, p. 3586-3597, Vol. 74, No. 8
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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