Herpesviruses

Unlike other DNA viruses such as papovaviruses and parvoviruses that use the cellular DNA synthetic machinery, herpesviruses encode most of the enzymes required to increase the pool of deoxynucleotides necessary for the replication of viral DNA. This facility is vital for viral replication in resting cells such as neurons, which throughout most of the life of the host never divide thus never synthesize DNA.

The HSV virion attaches via its envelope glycoproteins gC and gB to the heparan sulfate moiety of cellular proteoglycans, and then forms a firmer association between the envelope gD glycoprotein and one of further cellular receptors—herpesvirus entry mediator (HEM) or nectin-1.

Entry into the cytoplasm requires viral glycoproteins gB, gD, and gH and occurs by pH-independent fusion of the virion envelope with the plasma membrane in some cells (e.g., neurons) and via endocytosis in others (keratinocytes): the gB protein is responsible for fusion to the cell.

Tegument proteins are released, one of which (the UL41 gene product, an endonuclease) shuts down cellular protein synthesis. The capsid is transported by retrograde microtubule transport to a nuclear pore, where viral DNA is released, enters the nucleus, and is circularized.

The viral genome replicates by a rolling circle mechanism from one or more origins of replication. In the case of HSV-1, this involves at least seven virus-coded proteins—an origin-binding protein, an ssDNA-binding protein, a DNA polymerase composed of two subunits, and a helicase–primase complex composed of three gene products. Following DNA replication, certain β proteins induce the program of transcription to switch templates once more, and the resulting “late” (γ) mRNAs are synthesized and translated into the γ proteins, most of which are structural proteins required for morphogenesis of the virion.

Following DNA replication, certain β proteins induce the program of transcription to switch templates once more, and the resulting “late” (γ) mRNAs are synthesized and translated into the γ proteins, most of which are structural proteins required for morphogenesis of the virion.

Reference:

• Fenner and White’s Medical Virology. 5th edition.

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