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Journal of Virology, November 2006, p. 10743-10751, Vol. 80, No. 21
0022-538X/06/$08.00+0     doi:10.1128/JVI.01050-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

In Vitro Synthesis of Minus-Strand RNA by an Isolated Cereal Yellow Dwarf Virus RNA-Dependent RNA Polymerase Requires VPg and a Stem-Loop Structure at the 3' End of the Virus RNA{triangledown}

Toba A. M. Osman, Robert H. A. Coutts,* and Kenneth W. Buck

Division of Biology, Faculty of Natural Sciences, Sir Alexander Fleming Building, Imperial College London, London SW7 2AZ, United Kingdom

Received 22 May 2006/ Accepted 9 August 2006

Cereal yellow dwarf virus (CYDV) RNA has a 5'-terminal genome-linked protein (VPg). We have expressed the VPg region of the CYDV genome in bacteria and used the purified protein (bVPg) to raise an antiserum which was able to detect free VPg in extracts of CYDV-infected oat plants. A template-dependent RNA-dependent RNA polymerase (RdRp) has been produced from a CYDV membrane-bound RNA polymerase by treatment with BAL 31 nuclease. The RdRp was template specific, being able to utilize templates from CYDV plus- and minus-strand RNAs but not those of three unrelated viruses, Red clover necrotic mosaic virus, Cucumber mosaic virus, and Tobacco mosaic virus. RNA synthesis catalyzed by the RdRp required a 3'-terminal GU sequence and the presence of bVPg. Additionally, synthesis of minus-strand RNA on a plus-strand RNA template required the presence of a putative stem-loop structure near the 3' terminus of CYDV RNA. The base-paired stem, a single-nucleotide (A) bulge in the stem, and the sequence of a tetraloop were all required for the template activity. Evidence was produced showing that minus-strand synthesis in vitro was initiated by priming by bVPg at the 3' end of the template. The data are consistent with a model in which the RdRp binds to the stem-loop structure which positions the active site to recognize the 3'-terminal GU sequence for initiation of RNA synthesis by the addition of an A residue to VPg.

* Corresponding author. Mailing address: Division of Biology, Faculty of Natural Sciences, Sir Alexander Fleming Building, Imperial College London, London SW7 2AZ, United Kingdom. Phone: 44 207 594 5364. Fax: 44 207 584 2056. E-mail: r.coutts{at}imperial.ac.uk.

{triangledown} Published ahead of print on 23 August 2006.

Journal of Virology, November 2006, p. 10743-10751, Vol. 80, No. 21
0022-538X/06/$08.00+0     doi:10.1128/JVI.01050-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.





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