Arthi Chakravarthy

Humans and Viruses
Human Biology 115A
Winter, 1999
Robert Siegel, instructor

Replication of Bunyaviridae

This section will focus on the nature of Bunyaviridae replication within a cell. What is known about viral replication comes from work on the Bunyavirus genus on vertebrate cells. There are three segments in the genome: L (large), M (medium) and S (small). The L segment codes for the L protein which is used as viral transcriptase and viral replicase. The M segment codes for a polyprotein that is cotranslationally cleaved to garner glycoproteins G1 and G2. The S segment codes for the N nucleocapsid protein.

An image of the L,M, and S segments of Hantavirus

Attachment and Entry
Viral proteins on the surface are used to bind to a receptor on the cell surface. Current research on the LAC virus has indicated that G1 proteins are more critical for binding to cells than G2 proteins. It has not been determined which cell receptors are used by the virus to gain entry into the cell. However, it is known that entry is dependendent upon acidic conditions.

As Bunyaviridae is segmented, three mRNA species are transcribed from the three segments. The RNA of bunyavirus is negative sense , except for phleboviruses and tospoviruses which are ambisense. Bunyaviridae have their own encoded polymerases which are used for transcription. The mechanism for transcription initiation has not fully been determined, but there is some evidence that similar to the flu virus, bunyaviruses, hanta viruses and phleboviruses steal caps from host cell mRNAs. However, unlike orthomyxovirus, transcription occurs in the cytoplasm of the cell rather than in the nucleus. As a result, it is hypothesized that caps are stolen from mRNAs that are derived from host messages within the cytoplasm. Secondary transcription, which occurs after replication, increases the amount of L, S and M mRNA segments. Most of the segments found in the cell due to secondary transcription are S. The the least amount of segments are M, with an intermediate amount of L segments formed.

The L and S segments of the genome are translated by free ribosomes. M segments are translated by membrane bound ribosomes. The L, S, and M segments are responsible for different structural and non-structural proteins. Primary glycosylation and processing of envelope proteins occurs in Golgi bodies.

Viral RNA is used as a template for both replication and transcription. Replicative intermediates of positive sense RNA are used to create more copies of the RNA for the progeny. Replicative intermediates are full length template strands that have 6-8 daughter strands emerging from them.

Assembly and Egress
The viral progeny are assembled within the Golgi apparatus of the cell. All of the viruses are given glycoproteins and undergo terminal glycolysation. In addition, the viral progeny adopt pieces of host membrane to form the envelope. The viruses bud into the Golgi cisternae and then are released from the cell by exocytosis.

This is an image of Hanta virus found in a person. The number of virus particles found in the picture is the result of replication.

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