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Journal of Virology, February 2001, p. 1744-1750, Vol. 75, No. 4
0022-538X/01/$04.00+0   DOI: 10.1128/JVI.75.4.1744-1750.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

RNA Triphosphatase Component of the mRNA Capping Apparatus of Paramecium bursaria Chlorella Virus 1

C. Kiong Ho, Chunling Gong, and Stewart Shuman*

Molecular Biology Program, Sloan-Kettering Institute, New York, New York 10021

Received 24 October 2000/Accepted 16 November 2000

Paramecium bursaria chlorella virus 1 (PBCV-1) elicits a lytic infection of its unicellular green alga host. The 330-kbp viral genome has been sequenced, yet little is known about how viral mRNAs are synthesized and processed. PBCV-1 encodes its own mRNA guanylyltransferase, which catalyzes the addition of GMP to the 5' diphosphate end of RNA to form a GpppN cap structure. Here we report that PBCV-1 encodes a separate RNA triphosphatase (RTP) that catalyzes the initial step in cap synthesis: hydrolysis of the gamma -phosphate of triphosphate-terminated RNA to generate an RNA diphosphate end. We exploit a yeast-based genetic system to show that Chlorella virus RTP can function as a cap-forming enzyme in vivo. The 193-amino-acid Chlorella virus RTP is the smallest member of a family of metal-dependent phosphohydrolases that includes the RNA triphosphatases of fungi and other large eukaryotic DNA viruses (poxviruses, African swine fever virus, and baculoviruses). Chlorella virus RTP is more similar in structure to the yeast RNA triphosphatases than to the enzymes of metazoan DNA viruses. Indeed, PBCV-1 is unique among DNA viruses in that the triphosphatase and guanylyltransferase steps of cap formation are catalyzed by separate viral enzymes instead of a single viral polypeptide with multiple catalytic domains.

* Corresponding author. Mailing address: Molecular Biology Proram, Sloan-Kettering Institute, 1275 York Ave., New York, NY 10021. Phone: (212) 639-7145. Fax: (212) 717-3623. E-mail: s-shuman{at}ski.mskc.org.

Journal of Virology, February 2001, p. 1744-1750, Vol. 75, No. 4
0022-538X/01/$04.00+0   DOI: 10.1128/JVI.75.4.1744-1750.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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