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Genome Structure

  • Tripartite genome characteristic of Bunyavirus family

  • Three single-stranded RNA genome segments - large (L), medium (M), and small (S)

  • The terminal nucleotides undergo hydrogen-bond base pairing to form panhandle structures and non-covalently closed circular molecules

  • RNA genomic molecules complex with nucleocapsid (N) proteins to form ribonucleocapsid structures

  • For infectivity, each virus must contain at least one L, M, and S segment

  • Segment Functions:

    • L segments encode viral RNA-dependent RNA polymerase

    • M segments encode two glycoproteins (Gn and Gc) in a single ORF

    • S segments encode N (nucleocapsid) and NS (nonstructural) proteins

Image from ViralZone ExPASy Bunyavirus Page

  • All three segments of a given virus have the same sequence at their 3' and 5' ends

    • These terminal sequences are highly conserved within genera (as seen in the table to the right) 

  • Sense of the segments depends upon which genus the virus belongs to; they can be either negative sense or ambisense

    • L segment: always negative sense

    • M segment: ambisense in Tospovirus 

    • S segment: ambisense in phlebovirus and tospovirus

Virion Structure

Image from Fields Virology, 6th Edition, Chapter 42: Bunyaviridae, Page 1249

  • Virions appear spherical or pleiomorphic

  • 80 to 120 nm in diameter

  • Lilpid bilayer envelope that is 5 to 7 nm thick

  • Surface glycoprotein projections of 5 to 10 nm

  • 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:

  • Viral attachment is mediated between cell-surface proteins and viral proteins Gc and Gn (with Gc generally being the primary attachment protein)

  • Orthobunyaviruses, hantaviruses, and nairoviruses enter via clathrin-coated vesicles

  • Uncoating

  • 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

  • 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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