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J. Virol. doi:10.1128/JVI.02778-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

A shared transcription termination signal on negative and ambisense RNA genome segments of Rift Valley fever, Sandfly fever Sicilian, and Toscana viruses

Special Pathogens Branch, Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases. Centers for Disease Control and Prevention, 1600 Clifton Road MS G-14 Atlanta, GA 30329, USA; University of California, Davis, School of Veterinary Medicine, Davis, CA 95616

^* To whom correspondence should be addressed. Email: snichol{at}cdc.gov.

	Abstract

The Phlebovirus genus (family Bunyaviridae) is a diverse group of arboviruses that cause disease syndromes ranging from mild febrile illness to hemorrhagic fever with high fatality. Although antigenically similar, these viruses differ by approximately 25% at the genome level, and their ecologies, including geographic range, preferred vector species, and hosts, vary considerably. In contrast to other ambisense viruses where RNA hairpin structures which serve as transcription termination signals are frequently found separating the opposite sense ORFs, no evidence of predicted high energy hairpin structures were found at the ambisense junctions of phlebovirus S RNA segments. However, a conserved sequence motif was identified on both negative and ambisense genome segments that functions as a transcription termination signal for the N, NSs and GPC mRNAs in 3 diverse phleboviruses: Rift Valley fever, Sandfly Sicilian and Toscana viruses. The exact termination of nascent virus mRNA molecules was determined by 3'RACE analysis. Surprisingly, analysis of the termini of mRNAs from both S and M segments of these 3 viruses revealed that transcription termination occurred immediately upstream of a conserved sequence motif with the general features of 3' C_1-3GUCG/a 5'. In contrast, no corresponding sequence motif was found in the L segments, and analysis indicates a "run-off" transcript approach to L mRNA termination. The absolute requirement of the identified transcription-termination motif was demonstrated by using a highly efficient RVF virus reverse genetics system to generate live recombinant virus with a S segment lacking the termination signal motif for the NP or NSs mRNAs, and showing that these recombinant viruses generated mRNAs that failed to terminate correctly.

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