Bunyaviridae

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BUNYAVIRIDAE

Bunyavirus structure La Crosse virus (Transmisson EM) CDC La Crosse virus (Negative stain EM) CDC

Ganjam virus (Transmission EM) CDC

Virion size: 90-100nm
RNA: Negative strand.
Genome size: Three segments: L (6 to 12 kb), M (2-4.9kb), S (1 to 3kb)
Envelope: Yes (from host cell plasma membrane)
Replication site: Entirely cytoplasmic (as is the case with most RNA viruses)
Name: From Bunyamwera, place in western Uganda where the first bunyavirus was isolated (in 1943)

The Bunyaviridae are enveloped, negative strand RNA viruses with the helical RNA is in three segments (tripartite) known as the large (L), medium (M) and small (S) segments. The segments have complementary ends and so hybridize together. They encode:

L: The polymerase (RNA-dependent RNA polymerase)
M: Surface glycoproteins (Gn and Gc which form a dimer)
S: Nucleocapsid (N) protein and a non-structural protein (NS)

There is no matrix (M) protein

In a negative sense RNA virus, the genomic RNA must be copied to positive sense mRNA that is read by the ribosomes to make proteins. In Bunyaviridae the polymerase binds to a promotor at the end of each segment and makes the mRNAs. Transcription is terminated by a strong hairpin at the end of each gene and the mRNAs are capped by L protein.

The polymerase also makes a positive sense replicative intermediate that is the same length of as the viral genome which is then copied back to negative sense genomic RNA.

In some Bunyaviruses*, the S segment is more complicated since it is ambisense. This means that the positive strand mRNA copy of the S segment codes for the NS protein in the normal way; however, the positive stand replication intermediate can also be transcribed to a smaller sub-genomic RNA that can be read in the opposite direction to make the NS protein.

* Phleboviruses and Tospoviruses