Question

How and where do many DNA viruses such as SV40 replicate in the host cell?

Answer 1

Simian virus 40 ( Sv40)

• Belongs to polyoma viruses group
• The virus was first isolated as a contaminanat from a monkey kidney culutur used for the production of poliovirus vaccine.
• Its genome is very small and can be easile modified for gene therapy purpose.
• Its genome has ds circular DNA wrapped up as nucleosome with the help of histone proteins.

Viruses life cycles

• Virus binds to cell membrane and releases its DNA into host cell.
• replicated, transcribed and translated.
• Makes proteins necessary to make protomeres
• New virions are produced -lysis-lytic pathway
• Lysogeny- no lysis- incorporate viral DNA into host chromosome. Eg: Simian virus40( SV40)

SV40 replication in host cell

SV40 viruses are small (~40nm diameter), icosahedral, non-enveloped viruses that replicate in the nucleus.

Depending on the host cell, they can either transform the cell or replicate the virus and lyze the cell.

LYTIC CYCLE

Attachment, penetration and uncoating

Viral capsid proteins interact with cell surface receptors and penetration is probably via endocytosis. Virions are transported to the nucleus and uncoated. DNA (and associated histones) enters nucleus, probably through a nuclear pore
Production of viral mRNAs and proteins

Gene expression is divided into early and late phases.

• Early genes encode enzymes and regulatory proteins needed to start viral replication processes.
• Late genes encode structural proteins, proteins needed for assembly of the mature virus.

EARLY PHASE OF THE LYTIC CYCLE

Early gene expression

The early promoter is recognized by host RNA polymerase II (SV40 contains a strong enhancer). Post transcriptional RNA modification (capping, methylation, polyadenylation, splicing etc.) is carried out by host enzymes.

The early transcript (primary transcript) is made and then undergoes alternative processing, resulting in the mRNAs for the small T and large T antigens (these proteins have common amino-termini but different carboxy-termini).

The mRNAs are translated in the cytoplasm.

Note: Primary transcripts which can be processed and code for more than one protein are seen in several virus families and in the host cell.

LATE PHASE OF THE LYTIC CYCLE

The late phase starts with the onset of viral genome replication

DNA replication

• SV40/polyoma DNA replication occurs in the nucleus.
• Large T antigen is needed for DNA replication. This binds to the origin of replication.
• Polyoma viruses use the host cell DNA polymerase, which recognizes the viral origin of replication if the T antigen is present.
• DNA replication is bidirectional (There are two replication forks per circular DNA genome and replication involves leading/lagging strands, Okazaki fragments, DNA ligase, etc.).
• This process of DNA replication is very similar to that which occurs in the host cell - which is not surprising as the virus is using mainly host machinery except for the involvement of the T antigen.
• Host histones complex with the newly made DNA.

Late gene expression

• Late mRNAs are made after DNA replication (a lot of newly made viral DNA is now available as template). Early mRNAs are still transcribed, but at a very much much lower rate.
• T antigen is involved in controlling increased transcription from the late promoter and decreased transcription from early promoter.
• It also interacts with host proteins and changes the properties of the host cell, thus playing a role in cell transformation and tumor formation

Large T-antigen

• functional polyomavirus protein
• Binds and degrades cellular tumor supressors- promotes S-phase entry
• Binds viral replication origin, regulatory elements and cellular repliction machinery- perpetuates synthesis of large number of progeny and induces cell lysis.

Origin of replication in Sv40

SV40 ori adjacent to trnscription control region.

Initiation of replication needs vial large T antigen (major product of early trancription) binding to:

• Region within 64-bp ori core
• Two adjacent sites

T antigen helicase activity opens up replication bubble within ori core

Priming carried out by primase associated with the host DNA polymerase alpha.

SV40 Infection cycle

1. entry and release of core

2. Entry of DNA/ nucleosome complex to nucleus

3. Synthesis and alternative splicing of early(T) trancripts: proteinssynthesized

4. LT imported back to nucleus where it initiates DNA synthesis with host enzymes and potentiates late gene trnscription

5. Late gene mRNAs sythesized,spliced, exported, translated and products imported back to nucleus for particle assembly and release by unknown mechanism.