Genome Structure
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Tripartite genome characteristic of Bunyavirus family
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Three single-stranded RNA genome segments - large (L), medium (M), and small (S)
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The terminal nucleotides undergo hydrogen-bond base pairing to form panhandle structures and non-covalently closed circular molecules
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RNA genomic molecules complex with nucleocapsid (N) proteins to form ribonucleocapsid structures
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For infectivity, each virus must contain at least one L, M, and S segment
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Segment Functions:
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L segments encode viral RNA-dependent RNA polymerase
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M segments encode two glycoproteins (Gn and Gc) in a single ORF
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S segments encode N (nucleocapsid) and NS (nonstructural) proteins
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Image from ViralZone ExPASy Bunyavirus Page
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All three segments of a given virus have the same sequence at their 3' and 5' ends
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These terminal sequences are highly conserved within genera (as seen in the table to the right)
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Sense of the segments depends upon which genus the virus belongs to; they can be either negative sense or ambisense
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L segment: always negative sense
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M segment: ambisense in Tospovirus
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S segment: ambisense in phlebovirus and tospovirus
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Virion Structure
Image from Fields Virology, 6th Edition, Chapter 42: Bunyaviridae, Page 1249
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Virions appear spherical or pleiomorphic
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80 to 120 nm in diameter
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Lilpid bilayer envelope that is 5 to 7 nm thick
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Surface glycoprotein projections of 5 to 10 nm
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Interior appears coiled, beaded, or filamentous, likely due to ribonucleocapsids
A: Hantaan virion (Hantavirus​ genus); B: Puumala virus (Hantavirus genus); La Cross virus (Orthobunyavirus genus)
Image from Fields Virology, 6th Edition, Chapter 42: Bunyaviridae, Page 1251
Viral Replication Strategy
Special Features:
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Viral attachment is mediated between cell-surface proteins and viral proteins Gc and Gn (with Gc generally being the primary attachment protein)
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Orthobunyaviruses, hantaviruses, and nairoviruses enter via clathrin-coated vesicles
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Uncoating
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Transcription of viral negative sense (primarily) RNA to mRNA is done by viral RNA-dependent RNA polymerase, encoded by the L protein; this transcribes all three segments
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mRNA synthesis primed using cap-snatching to obtain nucleotide segments from host mRNAs in the cytoplasm (which is where viral replication occurs)
Image from Fields Virology, 6th Edition, Chapter 42: Bunyaviridae, Page 1254
Selected Page Bibliography
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"Bunyaviridae." ViralZone ExPASy Bioinformatics Resource Portal. http://viralzone.expasy.org/all_by_species/82.html
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Fields Virology, 6th Edition, Chapter 42: Bunyaviridae